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Volume XIX No. 6
February 2008
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JOHN TEBBUTT
By Mike Luciuk
The Ultimate Amateur Astronomer
J
ohn Tebbutt was a major astronomical force in
Australia during the nineteenth century. His
discovery of the Great Comet of 1881, C/1881
K1, coincided with advances in photography,
telescope drives and spectroscopy, and will be highlighted in this article. This
paper heavily paraphrases
papers by W. Orchiston and
S. Yousef, as listed in the
reference.
Introduction
John Tebbutt's interest in
astronomy knew no bounds.
Although this Australian's
fame was based on comets,
Tebbutt made his mark in
observations of variable
stars, the planets, and
eclipses, as well as on lunar
occultations and transits of
Mercury and Venus. His first
major discovery was the
Great Comet of 1861. He
plotted over 700 asteroid and
comet positions in his career.
He also discovered Nova
V728 Scorpii, maintained a
local time service, a meteorological station and carried
out studies on tides and floods. He actively popularized astronomy, published two books and was the
author of over 300 scientific papers. His accomplishments led to offers for important professional positions, which he declined. Tebbutt valued the freedom
his amateur status gave him to pursue astronomical
interests of his own choosing.
Tebbutt was very precise in his observational methods and record keeping. He was the first Australian to
systematically monitor variable stars on a long-term
basis. His observations of eta Argus (Carina) extended over four decades documenting its magnitude
variations. R Carina was covered over 22 cycles, permitting the accurate determinations of this Mira variable's period, maxima and minima.
Tebbutt was educated in several church schools, with emphasis on the classic languages
and Euclidean geometry. He
inherited his family's Windsor,
New South Wales farm, and
built his observatories there.
Over a period of time, it consisted of four separate buildings.
As with the buildings themselves, the instruments housed
became more expensive and
elaborate as Tebbutt gained the
means to improve them. Beginning with two telescopes, two
timepieces, a sextant and an
artificial horizon, Tebbutt furbished the observatories with
larger aperture instruments,
culminating in an eight inch
equatorially mounted refractor
by Grubb of Dublin - the observatory's largest main instrument.
Comet C/1881 K1, the Great Comet of 1881
In Tebbutt's astronomical memoirs, he notes his discovery of Comet C/1881 K1:
On going out this evening [1881 May 22] at 6h 15m as I
had done several times lately to scan the sky for comets
with the naked eye I detected what appeared to be a
nebulus [sic] object in the south west a few degrees
above the place where I discovered the great comet of
1861. I saw at a glance that it was a new object as I am
very familiar with that part of the heavens.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
Subsequently, Tebbutt observed Comet C/1881 K1
through to June 13, making 68 positional measurements with a filar micrometer, on 13 different dates. A
filar micrometer accurately measures the angular
separation and relative orientation of two neighboring
astronomical objects. He also recorded useful information about the tail and the head (but particularly the
changing nature of the nucleus), and determined the
apparent visual magnitude of the comet on a number
of occasions. He published his first report on the
comet for the international astronomical fraternity in
Astronomische Nachrichten, and followed this up with
further positions. Tebbutt then devoted the next three
months to calculate the comet's orbital elements. This
difficult task took 40 manuscript pages to complete.
also due to the film and telescope drive improvements.
Huggins described the spectrum as follows:
The spectrum of the comet consists of a pair of bright
lines in the ultra-violet region, and a continuous spectrum
which can be traced from about F [486 nm] to some distance beyond H. The bright lines, a little distance beyond
H, with an approximate wave-length from 3870 to 3890,
appear to belong to the spectrum of carbon (in some
form, possibly in combination with Hydrogen), which I
observed in the spectra of the telescopic comets of 1866
and 1868. In the continuous spectrum shown in the photograph, the dark lines of Fraunhofer can be seen.
Note that the K, H, h and G Fraunhofer lines refer to
wavelengths of 393 nm, 397 nm, 410 nm and 431 nm
respectively. The bright lines near 388 nm and 420 nm
are likely the molecular spectral lines of cyanogen,
(CN)2.
Tebbutt's comet was characterized by astronomers
as structurally complex with a rapidly changing head,
and an intricate tail. It was noted that:
At one time ... a separation in the nucleus led astronomers to think that it would follow the example of
Biela's Comet, but the following night the smaller part
had disappeared. Very frequent changes were observed
by the telescope, in the shape of the jets and envelopes
around the nucleus. Many drawings were made of it.
The John Tebbutt Observatories, Windsor N. S. W.
As the Great Comet began to appear in the Northern Hemisphere, it drew a great deal of attention. By
the end of June, its tail exceeded 25 degrees in length
and its coma was magnitude +1. By this time improvements in dry gelatin plates and telescope drives
made it possible, for the first time, to photograph a
comet including its tail. C/1991 K1's photograph was
taken by Janssen on June 30, 1881.
Also, Comet C/1881 K1 was the first comet whose
spectrum was successfully photographed. All prior
As for C/1881 K1's tail:
... a curious dorsal spine of strong illumination formed the
axis of the tail, which extended in clear skies over an arc of
20°. It belonged to the same "type" as Donati's great plume
... the appendage was, for a few nights, and by two observers, perceived to be double. Tempel, on June 27, and Lewis
Boss, at Albany (N.Y.), June 26 and 28, saw the long
straight ray [and] ... the curved train ... It had vanished by
July 1, but made a temporary reappearance July 22.
All in all, Tebbutt's comet was indeed, one of the
Great Comets of the nineteenth century.
Spectrum of Tebbutt's Comet C/1881 K1
cometary spectra were obtained visually. On June 24,
Sir William Huggins made a one hour exposure of the
Great Comet's spectrum, the success of which was
2
(Continued next page)
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
Final Points
The Great Comet of 1881 was discovered by John
Tebbutt almost twenty years to the day after he detected the Great Comet of 1861. Both comets were
spectacular objects and contributed to our understanding of cometary structure and behavior, but because of the emergent role of photography and spectroscopy in astronomy, the 1881 comet was to play a
more notable part in the development of cometary
astronomy.
At Left: Comet C/1881 K1 discovered by
John Tebbutt
In 1973 Tebbutt's contributions in astronomy were
recognized when a crater on the Moon was renamed
"Tebbutt." In 1986 the Australian government issued
their $100 note with John Tebbutt's image overlaying
his observatories. These were fitting tributes to this
consummate amateur astronomer.
Australian $100 note
References
Orchiston, W., 1999. C/1881 K1: A Forgotten “Great Comet” of the Nineteenth Century. Irish Astronomical Journal, 26(1), 33.
Orchiston, W., 2000. John Tebbutt of Windsor, New South Wales: A Pioneer Southern Hemisphere Variable Star Observer. Irish
Astronomical Journal, vol. 27, iss. 1, p. 47.
Tebbutt, John, 1881. Appearance of Another Southern Comet (comet b 1881). The Observatory, Vol. 4, p. 239-240.
Yousef, S., http://progsoc.org/~sammy/astronomy/AIMPapers/Tebbutt%20Observatory.pdf
Are You Among the Five Percent?
It seems like five percent of the people in every crowd just don’t get the word. At AAI, we try
to give you lots of ways in which to keep in touch. This newsletter and the AAI website and
all the forwarding addresses listed on page 15 below are good media, but are you sure your
contact info including your primary and alternate phone number and email address are updated in the Membership database? Are you using AAI’s free discussion groups (aaigroup and
AAI-Soapbox) on Yahoo? Are you included in the password-protected email list on the “AAI
People” page of the website? Send an email to [email protected] and we will help
you stay connected.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
3
Dr. Julius Zsako 1920 - 2008
A note from Dr. Lew Thomas:
Julius Zsako, a long standing AAI Life Member and Sponsor and past Membership Chairman, died
on Tuesday, January 29, 2008.
Julius was born in March, 1920 in Transylvania which was originally a part of Hungary and is now in
Romania. As a youth, he was greatly involved in school sports and he attended the so called Gymnasium in Hungary. At the outbreak of World War II, he was drafted into the Hungarian army then
controlled by Germany. After the war, he left Hungary and moved to various places in western
Europe. At war's end, Hungarians were to be repatriated to Hungary. Julius did not want this since
Hungary was then controlled by Russia. He moved around Europe and finally gained passage to the
United States. Here he learned English, found work, and became an American citizen. He worked as
a photographer, and was employed in a New York bank. He obtained a doctorate degree from Columbia University adding to the one he had received in Hungary. Thirty four years ago, he joined
Seton Hall University and became head of its Music Department as a full professor.
During the years before Seton Hall he aided hundreds of Hungarians enter United States, to gain
employment, and to become citizens. That was the nature of Julius: to help others. Long after their
graduation, many students would greet Julius and thank him for his teaching and caring. Julius is still
helping others for he has donated his body to science.
Julius is survived by a niece and other family members in Hungary. A memorial is being planned at
Seton Hall on February 15th (see below).
I first met Julius shortly after he joined AAI in 1983, and I remember well that first meeting. I had just
printed a section of a sky map centered around Coma Berenices. I proudly showed it to Julius who
pointed to a star and stated that is no star at that location. Of course, I looked it up, and, to my
amazement, Julius was right! Here indeed was a true amateur astronomer.
I have met a few great people in my lifetime. Julius was one of these.
________________________________________________
Memorial Service Seton Hall University
Friday, 15 February 2008 at 2:00pm
Seton Hall University
400 S Orange Ave, South Orange, NJ 07079
Jubilee Hall, Top Floor
Click http://tinyurl.com/2qbsog for interactive map.
Ask for parking at Security Booth on campus.
Click http://tinyurl.com/35dvu9 to see the obituary that was published in the Star Ledger on
February 3rd.
4
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
Stewart’s Skybox
s usual, February is a boring month. About
the only major astronomical event is the lunar eclipse on the night of the 20th, but the
topic of lunar eclipses has already been covered. However, some recent news on the Space
Daily website (http://www.spacedaily.com) has suggested a topic.
Last year, I polled the membership about what
topic they wanted in one of the columns. Alex Flynn
had a suggestion about computer-controlled telescopes and won. But there was one suggestion that
was quite good as well and that will be the topic of
this column, especially in light of some recent findings.
The suggestion is from our own multi-talented Clif
Ashcraft. He wanted to hear about jets. No, not those
sardine cans with wings that fly all over and used to
ruin many a long exposure astrophoto (back when
people used film to photograph the night sky).
Rather, I am talking about the cosmic variety.
A
Starting Big
In the early 20th century, improvements in photographic technology allowed astronomers to take images of relatively faint objects through their telescopes. In 1918, Heber Curtis took a picture of M87,
an elliptical galaxy in Virgo, that is well known to
many amateurs. When the plate was developed, he
noticed what he called "a curious straight ray" sticking out of the nucleus of the galaxy.
Little progress was made toward explaining this
phenomenon until the 1950s. As a result of the pioneering insights of Karl Janssky and Grote Reber in
the 1930s being combined with technology derived
from radar research in World War II, radio astronomy
entered the astronomical mainstream. In those days,
astronomers were merely detecting sources of radio
emission and establishing their approximate locations. One of the brightest sources was in the constellation of Virgo and was named “Virgo A”. As the
resolution of radio telescopes improved, the source
of the emission was narrowed down to M87, then
eventually to the "curious straight ray".
With the improved radio and optical data, this "ray"
was shown to be a jet of gas streaming away from
the nucleus at very high speeds, as is very clearly
shown in the Hubble Space Telescope image from
the Space Telescope Science Institute in Figure 1.
Astronomers were puzzled and could not figure
out how a confined jet of material could be shot away
from a galactic nucleus. Over the years, more of
these jets were found in other galaxies.
by Stewart Meyers
As knowledge of galactic jets was gradually improving, a clue was provided by the study of quasars. Discovered in the early days of radio astronomy, these objects were considered very mysterious,
though by the 1960s, it had been found that they
were in the centers of very distant galaxies. However, news of this was slow to spread outside the
Fig. 1 HST image of M87's jet
http://imgsrc.hubblesite.org/hu/db/2000/20/images/a/formats/web.jpg
astronomical community. In fact, in the original "Star
Trek" series episode "The Galileo 7", it was maintained that they were mysterious objects in our own
galaxy. In defense of "Star Trek", revisionist astronomer Halton Arp also believed that quasars were
much smaller and closer than other astronomers
thought.
While studying quasars, some astronomers noticed evidence of gas moving at very high speeds
away from a few quasars. In fact, the speeds were
too high - faster than light. It was soon realized that
this superluminal speed was just an illusion caused
by relativistic effects when the galactic jet from the
quasar is almost pointed at Earth and is moving at a
substantial fraction of the speed of light.
For some time, it was thought that jets were found
only within active galactic nuclei. But that would
change.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
5
ters of galaxies. It was also realized that young galSS433 and SNL
axies have large quantities of gas in their central
In 1969, it was found that some X-ray sources in
regions, which would eventually fall into the central
our galaxy were highly variable. But there was a surblack hole and form an accretion disk. So SS433
prise in store. Bruce Margon, in 1978, studied the
offered a link between the jets emitted by galaxies
spectra of an X-ray source in a supernova remnant
and quasars and those emitted by neutron stars in Xknown as W50 in Aquila. He noticed that the Doppler
ray binaries and black holes.
shift of the signal changed over time. It was red
shifted, normal, then blue shifted. This cycle would
repeat. Further investigation found that the source
White Dwarf
It has been recently discovered that white dwarf
was a binary system with a 13-day orbital period
stars, the end result for stars like our Sun and
which was emitting jets of gas and particles and also
smaller, can, in some cases, get into the cosmic jet
that the jets had a precession cycle of 160 days. As
act just like
usually hapneutron
pens with
stars and
astronomical
black holes.
discoveries,
Suzaku, an
when word
X-ray satelgot out to the
lite operated
mainstream
by the Japamedia, they
nese Aeromisinterspace Expreted the
ploration
information
Agency
and thought
(JAXA) and
that there
NASA's
was a star
Goddard
that was both
Space Flight
coming and
Center, obgoing at the
served AE
same time.
Aquarii in
In what
2005 and
was a first in
2006. AE
the history of
Aquarii was
astronomy,
Fig.
2
Image
of
M57
(The
Ring
Nebula)
taken
by
AAI
Member
Hank
Adams
on
known to
this X-ray
August 10th, 2005 with an SBIG ST8XE CCD camera. Image is a composite of
have a very
source, now
red, green, and blue exposures of 150 seconds each.
strong magknown as
http://www.asterism.org/gallery/gifs/m57-04.jpg
netic field,
SS433, was
and it is in a
featured in a
binary sysbrief segment
tem with a main sequence star. Yukikatsu Terada of
on the "Weekend Update" skit on "Saturday Night
the Institute of Physical and Chemical Research
Live" where Don Novello (as his famous character
(RIKEN) in Wako, Japan used Suzaku to observe AE
Father Guido Sarducci) claimed that study by the
Aquarii in an effort to determine that white dwarf
astronomers at the Vatican Observatory revealed
stars with strong magnetic fields could generate
that everything in that star system went forwards,
cosmic ray particles. To the surprise of the scientists,
then backwards, including the aging process.
AE Aquarii acted like a pulsar, emitting a jet of from
When studied by real astronomers, it was deterthe accretion disk of material that the white dwarf's
mined that SS433 was what is known as an X-ray
gravity pulled from its main sequence companion.
binary. That is a normal star, usually in the red giant
phase, in close association with a compact object. In
the case of SS433, the object is a neutron star. The
Small Scale Version
Back in 1890, S.W. Burnham discovered a small
neutron star pulls gases from the outer atmosphere
nebula in Taurus, near a variable star that is now
of its companion, and forms an accretion disk.
known as T Tauri. At the time, it was known as
Somehow jets of material shoot out from the system.
Burnham's Nebula because it was considered to be
Around this time, astronomers were starting to aca small example of a reflection nebula. Then, in
cept that very massive black holes exist in the cen-
6
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
It seems that all cosmic jets have a few things in
1946, two astronomers, George Herbig and the late
common. First, they all involve objects (black holes,
Guillermo Haro, were independently studying NGC
neutron stars, white dwarf stars, or protostars) that
1999 and Burnham's Nebula when they noticed
are surrounded by much larger disks of material.
some bright objects in their images. These bright
Secondly, the material is falling towards the object.
objects were soon known as Herbig-Haro objects
Finally, the central object has a magnetic field.
and were considered somehow connected to the
These common elements are the crucial clues to
process of star birth. Based on the evidence of early
how jets form.
spectroscopic work, it was thought for a time that
The current view is that matter is spiraling in tothese objects contained newborn stars, being flung
wards the central object. As a result, it forms a flatout of the nebula where they were born. But the actened disk and heats up due to internal friction, betual truth was much different.
coming hotWith imtest close to
proved resothe central
lution, the
object where
Herbig-Haro
the accretion
objects were
disk is rotatrevealed to
ing most rapbe the reidly. Some of
sults of jets
this heated,
shooting out
rapidly movof the nebing matter
ula where
becomes
stars were
ionized and
forming and
interacts with
hitting a
the central
dense outer
object's magpart of the
netic field
nebula. The
and is carried
resulting
away from
shock
the disk in a
causes the
magnetically
gases to
contained jet.
ionize and
Recently, this
glow. At
role of magfirst, asnetic fields in
tronomers
Fig. 3 HST image of planetary nebula Hubble 5
cosmic jets
were not
http://imgsrc.hubblesite.org/hu/db/1997/38/images/f/formats/web.jpg
received masure why
jor confirmanewborn
tion from work done by a team lead by Rodrigo
stars should be emitting jets, since the prevailing
Nemmen of Penn State University. They studied
theories indicated that star birth was a gradual and
Chandra observations of nine galaxies known to
relatively sedate process. However, it has been
harbor supermassive black holes emitting jets. When
learned that is not the case. The disk surrounding
factoring in relativistic effects with new information
newborn stars is not a simple, peacefully rotating
concerning rotation and the magnetic properties of
object that gradually accretes onto the star. But
black holes, they found that the combination resulted
rather it is a place where gas and dust are in conin an extremely powerful magnetic field that was able
stant motion, with some of the material spiraling in
to eject a large fraction of the matter falling towards
toward the star.
the black hole before it could get close to the event
horizon. This material is confined in jets shooting out
Bringing It All Together
from the magnetic poles. The same process - on a
Seeing jets shoot out of active galaxies is one
much smaller and non-relativistic scale - is thought
thing, but when jets are seen associated with other
be responsible for other cosmic jets as well. This
kinds of astronomical objects, that is significant, and
confirms that magnetic fields are absolutely required
it can tell us something about how the universe
for the creation of cosmic jets.
works.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
7
Close But Not Quite
In recent years, a new jet-like phenomenon has
been discovered. When red giants approach the end
of their lives, they shed the outer layers of their atmosphere, which then forms what we call planetary
nebulae. It was thought that this process was spherical. Shapes of planetary nebulae such as M57 (the
Ring Nebula) were presented as evidence of this.
See Figure 2.
But, when telescopes like the Hubble Space Telescope took images of planetary nebulae, some had
shapes like butterflies, others had shapes suggesting
two lobes, such as seen in the HST image of Hubble
5 in Figure 3.
While this phenomenon is not totally understood, it
is thought that some of the stellar wind is somehow
channeled so that much of it goes out perpendicular
to its equatorial plane. While not a true jet like the
others mentioned in the article, it is another case
where gas flow is directed along an axis.
One Process Fits All
This amazing process works in a wide variety of
environments, from the large scale of active galactic
nuclei with multi-million solar mass black holes all
the way down to protostars at the beginning of their
lives, and, to a limited extent, even to dying red giant
stars.
Apparently, nature likes to use the same rules
over and over again.
A cookie in the mud? A fossil in the tar pit? No, it’s the APOD for February 11, 2008
http://www.stumbleupon.com/demo/#url=http://antwrp.gsfc.nasa.gov/apod/ap060620.html
8
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
The Hubble Age
By Dr. Lew Thomas
T
he farther away a galaxy is from us, the greater is the red shift. Hubble and Humason reported this by
constructing a graph which plotted red shifts, or recessional velocities, against distances to each galaxy
measured. See Figure 1. This work began back in 1924, when Hubble began to use the newly fabricated 100-inch telescope at Mt. Wilson in California. Humason was then driving a mule train to haul equipment
and astronomers up a dirt road leading to the summit. Humason later became what amounted to be a janitor at
the observatory. His curiosity earned him a position as assistant to the nighttime astronomers, helping them
point the huge telescope to collect their data. Humason absorbed information so quickly and learned skills so
rapidly that he finally became a full fledged astronomer at Mt. Wilson. This is a success story that shows what
can be accomplished when one is highly motivated.
These two astronomers concentrated on galaxies in clusters to which the distances could be
estimated by assuming that each cluster was
made up of about the same variety of galactic
types. The more galaxies in each group, therefore, the brighter was the assemblage. By the
inverse square law, the distance could then be
estimated independently of the red shift. When
the red shift vs. distance plot was made, it was
observed that the more distant a galaxy was, the
greater was its red shift.
The slope of the velocity (red shift number) vs.
distance curve is the Hubble constant (H). The
curve was assumed to be a straight line since the
expansion rate was believed to be linear. The
Hubble constant is usually expressed as km/sec
per megaparsec. If the slope is really constant
over eons of time, the Hubble constant gives a
measure of the age of the universe. Let this age
be To and so,
To = 1/H
(1)
19
Now a megaparsec equals 3.086x10 kilometers
and there are 3.15576x107 seconds in a year, so
that for:
H = 1 km/s/megapc
1 km/s/mpc = (1 km/sec) / 3.086x1019 km
= 3.240x10-20 / sec
= 3.240x10-20 x 3.15576x107 = 1.0268x10-12 / year
= 1.0268x10-12 x 109 = .001027 / billion years
Fig. 1 Hubble's distance-velocity relationship for galaxies based on his original data. This is now known as
Hubble's Law and is interpreted as evidence for an expanding Universe.
http://outreach.atnf.csiro.au/education/senior/cosmicengine/im
ages/cosmoimg/hublaw1.gif
(2)
If H = 80 km/s/megapc, from (1) and (2) we obtain
To = 1 / (80 x 0.001027) = 12.1 billion years
(3)
This is called the Hubble Age and, as we have said, is based upon a constantly expanding universe.
We can simplify the arithmetic by revising (3) to express
1/H = 973.7 billion years per 1 km/sec/megaparsec
(4)
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
9
and so, To = 973.7/H ; 1000/H billion years
(5)
where
H = number of km/sec/megaparsec.
Since the expansion of the universe is accelerating, then the universe must be older than the Hubble Age.
We hypothesize that dark energy, a repulsive force, is pulling the universe apart. If this energy be constant
over time, then as the universe expands, the attractive force of gravity will be reduced in accordance with the
inverse square law, and so the acceleration itself must increase without limit. To date, it has been determined
that the acceleration itself has two measures of acceleration (two derivatives). It awaits more data to determine
if there are any more derivatives. If there are not, then dark energy can not be constant, but must decrease
slowly with time.
Origin And Use Of The Julian Day Number
by Dr. Lew Thomas
Julian Day Numbers are the consecutive count of days from noon at the Greenwich meridian on January 1,
4713 BC. There are algorithms to convert any date to a JDN and vice versa. This is particularly important in
astronomical work since it avoids months, years, and leap years in our modern calendar. By agreement all
dates prior to October 5, 1582 are calculated on the basis of the Julian Calendar containing 365.25 days and
having a leap year every 4 years. Dates following October 5, 1582 are calculated on the basis of the Gregorian
Calendar having leap years every 4 years with the exception of century years (those ending in 00) in which
case a leap year occurs if the date is evenly divisible by 400. Due to the calendar conversion from the Julian to
Gregorian calendar the dates of October 6 through October 15, 1582 do not exist.
The count of days by Julian Day Numbers was invented by Joseph Scaliger in the 15th century and published in 1583. The word "Julian" in the JDN comes from the fact that 365.25 days in the Julian Day system is
considered a year as in the Julian calendar. It must be noted, however, that the Julian Day Number is simply a
count of days from noon on January 1, 4713 BC.
In doing his work, Scaliger consulted over 50 calendars of his day in order to convert their dates to JDNs.
Now the question is why did he choose as the staring point for the count the year 4713 BC? He considered
three cycles which were commonly employed in reckoning dates in Scaliger's time, namely
1) the 28 year solar cycle - this is the shortest period in which the days of the week return to the same calendar day in the Julian calendar.
2) the 19 year lunar cycle - this is the Metonic cycle in which the same phase of the moon returns to the
same Julian Calendar date
3) the 15 year cycle of the Roman Indiction - a political cycle.
When one combines these three cycles one obtains a grand cycle of 28x19x15 = 7,980 years.
Now comes the religious part of the puzzle. Scaliger inferred that the birth of Jesus was in 1 BC (which most
modern scholars deem as incorrect). Each of the above cycles began at a different epoch and those near 1
BC were
1)
2)
3)
1 BC was the 9th year of the solar cycle
1 BC was the 1st year of the lunar cycle
1 BC was the 3rd year of the Roman Indiction
Now Salinger sought a day count which would always be positive, so he needed a time that preceded all recorded history.
(Continued page 14 Julian Day Number)
10
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
Happy Leap Year!
As you may already know, February 2008 will
have 29 days. To most people the primary significance of this fact, like many others, is monetary: if
you're paid by the hour, that's an extra day for which
you get paid, but you work an extra day for free if
you get paid monthly (yet you also get one free day
on your monthly rent). For the approximately 0.1% of
people who were born on February 29th (like The
Asterism editor Ray Shapp's grandfather), it means
many less "official" birthdays.
But what is the astronomical significance of the
leap day, and what is the reasoning behind the labyrinthine rules governing whether any particular February will have a 29th day? For example, 2008 will
have a leap day, as did 2004, 2000, 1996 and every
fourth year back to 1904. But 1900 did not (at least in
places where the current calendar was in use), although every divisible-by-four year from 1986 back
to 1804 did, and neither did 1800 or 1700 (yet all the
other intervening divisible-by-four years did have 29
days in February). 1600, on the other hand, was a
leap year, unlike the three succeeding century years.
2100, 2200 and 2300 will not be leap years, but 2400
will be. Are the calendar designers trying to torture
us?
The fundamental need for the leap day arises from
the fact that the year does not correspond to an integer (whole-number) multiple of days. If one defines
the year as simply being 365 days, the Earth will not
yet have returned to the same point in its orbit after
one year. This effect will add up over time, and eventually the seasons will be completely out of phase
with the calendar! Happy snowy Fourth of July!
The immediately obvious solution, of course, is to
add a fractional day to the end of every year, giving
the Earth extra time to "catch up." The problem is
that the insertion of whatever number of extra hours
between 11:59pm December 31st and 12:00am
January 1st will cause the Sun's position to be "out of
phase" with the clock on the wall (a year of Sunrises
at 1am, or 7pm, or 11:45am…), an even more immediately distressing problem! So this is not an acceptable solution; to keep our clocks and the Sun in
phase, any "catch-up" time must be added in multiples of a whole day.
What we must do is come up with a scheme to
have extra days on certain years and not on others,
so that one "average" calendar year closely approximates the true year, and the long-term drift of seasons is cancelled out.
But first we should agree what it means to talk
about "days" and "years." We all learned in school
By Jeremy Carlo
that a "day" is the amount of time it takes the Earth to
rotate about its axis and a "year" is the amount of
time it takes the Earth to go around the Sun. (According to a recent survey, nearly half of Harvard
graduates are aware of these facts.) But this simple
description glosses over some important points.
The Earth's rotation is indeed quite regular; this is
enforced by the conservation of angular momentum,
although perturbative effects, chiefly due to the
Moon, cause the Earth's rotation axis to slowly drift
("precess") and its rotation rate to gradually decrease. When the Ancient Egyptians built the pyramids, the "North Star" was Thuban (α Draconis), and
several hundred million years ago, the length of the
day was closer to 18 hours than 24 (less time for
Jack Bauer to save the world). At the present time, it
takes the Earth 23 hours, 56 minutes and 4.1 seconds to rotate once about its axis, relative to a stationary reference frame in space. This is known as a
sidereal day, and is useful to astronomers because
the sidereal time tells us the Earth's absolute rotational position and thus, for example, whether or not
Orion is well-placed for viewing, or when M31 will be
rising at a given location. Some hard-core astronomers even have sidereal clocks in their observatories.
But for farmers and fishermen and other practical
types, the Sun's location is far more important than
that of the Big Dipper or the Orion Nebula. Sidereal
clocks won't tell you that, and here's why. As the
Earth rotates, it also revolves about the Sun. When
the Earth completes one rotation in the sidereal
sense, it has moved about 1/365th of an orbit, and
thus the Sun has not, from our vantage point, yet
returned to quite the same place in the sky. So we
have to add a small amount of additional time (about
4 minutes, which is about 1/365th of a day), to allow
the Sun to "make it all the way around;" this is what
normal people call a "day," with 24 hours of 60 minutes each, of 60 seconds each, for 24x60x60 =
86,400 seconds, and we astronomers call a solar
day.
(As a side note, the amount of "extra" time required to get the Sun to return to the same point in
the sky varies due to the Earth's elliptical orbit;
sometimes the Earth moves faster than at other
times, and thus the "catch-up" time required is not
constant. 24 hours (the sidereal day plus the 3 minutes and 55.9 seconds) represents an average of the
Sun's actual motion through the sky, and is referred
to as a mean solar day. Put the Earth's elliptical orbit
together with the inclination of the Earth's rotation
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
11
axis, and if you take a photograph of the Sun at the
same mean solar time (say, noon) each day, over
the course of a year the Sun's position will trace out
the beautiful analemma, as explained by Alberto
Guzman in last month's Asterism.)
Now that we agree that a "day" is actually a mean
solar day of exactly 86,400 seconds, what about the
year? (There is another complete story about what
exactly a "second" is, giving rise to UT1, UTC and a
whole alphabet of time systems, and necessitating
the occasional insertion of "leap seconds," which I'll
save for a future article.)
The year, it turns out, is even more complex, as
the Earth, strictly speaking, never returns to the exact same point it occupied a year ago since its orbit
is elliptical and inclined with respect to the Sun's
equator. If we neglect this and just consider the time
taken for the Earth to move 360 degrees around the
Sun with respect to a fixed reference frame, this is a
sidereal year (current value: 365.256363 days, to six
decimal places). But, the perihelion (the location
where the Earth is closest to the Sun) drifts very
slowly, so the Earth will not be the same distance
from the Sun after one sidereal year; the time for the
Earth to go from one perihelion to the next is called
the anomalistic year (currently 365.259635 days).
Perihelion precession is itself a complex phenomenon, and is partly due to effects of general relativity
(Mercury's anomalously large perihelion precession
was an important early piece of evidence for Einstein's theory of General Relativity), also a topic for a
separate article.
But, what has the most importance for practical
people is the interaction between the Earth's rotational inclination of 23.5 degrees and the Earth's orbit; summer occurs when your hemisphere is pointed
towards the Sun, and winter occurs when your hemisphere is pointed away. (This also explains why the
seasons in the southern hemisphere are reversed
from ours.) Surprisingly, this inclination effect outweighs that of the Sun-Earth distance in determining
surface temperatures; perihelion occurs in January,
as the Northern Hemisphere is in its winter chill, and
aphelion (farthest distance from the Sun) is in July.
The vernal and autumnal equinoxes heralding the
northern hemisphere's spring and fall, respectively,
occur when neither hemisphere points more toward
the Sun than the other, while the summer and winter
solstices occur when your hemisphere is tilted
maximally toward or away from the Sun. The time
from one vernal equinox to another (more correctly,
the average of the time from any one such point to
the next) is known as the tropical year, and is currently 365.242190 days.
12
It may seem like this discussion is overly pedantic,
but it's important to note that the sidereal year and
the tropical year differ by about 20.4 minutes. This is
due to the same rotational precession that causes
the north celestial pole to wander. Therefore, the
position of the Earth around the Sun at the vernal
equinox will slowly move around with respect to the
stars; this is known as the precession of the equinoxes. (A somewhat whimsical consequence is the
fact that your "star sign" as published in horoscopes
is most likely wrong; the Sun was probably in a constellation that neighbors your "sign" at the time of
your birth due to the accumulated effects of equinoctal precession since the "charts" were first made.)
The most useful calendar is one that tells us that
the weather is hot and the Sun beams down from
high in July and the weather is cold and the Sun
hangs low in December, and tells us when to harvest
the crops, prepare for monsoons, or to put on the
snow tires. We should, therefore, match our calendar
as closely as possible to the tropical year, with every
year having an integer number of days. A simple
calendar with 365 days every year will "gain" about
one day every 4 years, probably not enough to be
readily noticeable over an individual lifetime, but inexorable on the timescales of history's long arc. Indeed in a little over a millennium the calendar will
have completely "lapped over" a seasonal cycle.
The first workable solution was developed in 46-45
BC, during the time of Julius Caesar. Since the little
bit extra above 365 is almost equal to a quarter
(0.25), why not add one extra day every fourth year?
This gave rise to the Julian calendar, which has
three years of 365 days to every one year of 366
days, averaging exactly 365.25000 days per Julian
year: every year whose number is divisible by 4 shall
have a leap day added to the end of February,
whereas those years not divisible by 4 shall have 28
days in February.
However, the tropical year is 0.007810 days less
than the mean Julian year. This discrepancy will accumulate as well, albeit more slowly: the Julian calendar "loses" one day with respect to the seasons in
about 130 years. What ultimately brought down the
Julian calendar was Easter, which is tied to Passover, and thus is based on lunar cycles rather than
the solar calendar. While the Roman Catholic
Church and the Church of Alexandria (today the Orthodox Churches) used slightly different standards,
both were dependent on the date of the vernal equinox and a preset "window" of dates on the calendar.
As the equinox drifted relative to the window of allowable dates, conflicts arose as to when Easter
should be held.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
A solution was proposed and then decreed by
Pope Gregory XIII in 1582, when the accumulated
drift of the vernal equinox added up to about ten
days; the Gregorian calendar was gradually adopted
by civil authorities and remains in use today. See
Figure 1. In switching from the Julian calendar to the
Gregorian one, calendars were advanced the appro-
colonial period, except for Alaska, which switched in
1867 when it was purchased from Russia. The first
correction the Gregorian calendar imposed was to
decree that century years (those ending in 00) are
not to be leap years; this reduces the length of the
Gregorian year from the Julian value of 365.250000
days to 365.240000 days, only differing from the
Fig. 1 Mismatch between seasons as a function of year. The small 4-year sawtooth cycles are caused
by having one leap year every four years; if there were no leap years the mismatch would very rapidly
accrue, as it would be a continuous line with a slope equal to that of any of the individual “teeth.”
Even the 4-year cycle has a slight mismatch, visible as the upward trend within each individual
sawtooth “set;” this effect is compensated by skipping one leap year each century, thus “resetting” the
height. There is still a small mismatch (the difference in the heights of 1804 and 1904, or 2104 and
2204, for example), which is compensated by having a leap day anyway once every 400 years (as seen
in the middle of the graph).
http://upload.wikimedia.org/wikipedia/commons/8/89/Gregoriancalendarleap.png
http://en.wikipedia.org/wiki/Gregorian_calendar
priate number of days to negate the accumulated
effects of precession under the Julian calendar.
Spain, Portugal and Italy went directly from October
4 to 15, 1582 when the switch occurred; France
skipped from December 9 to 20 of the same year.
Most of Protestant Europe (some fearing the Gregorian calendar as a tool of Catholic imperialism) did not
switch over until the 18th century, necessitating an
11-day skip, and Russia held out until the October
Revolution of 1917, skipping 13 days from January
31 to February 14, 1918. Most of what eventually
became the United States switched during the early
tropical year by 0.002190 days, about ¼ of the Julian
calendar's deviation.
The second correction was to stipulate that every
fourth century would have a leap day anyway, raising
the long-term average to 365.242500 days per Gregorian year, leaving a deviation of only 0.000310
days per year, meaning the Gregorian calendar will
"lose" one day per 3000 years.
It was felt at the time that this would not cause any
trouble over the timescales the calendar was expected to be used. However, it is an interesting exercise to determine what the next "correction" would
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
13
be, assuming of course we still used the Gregorian
calendar after the millennia, or if indeed there still is
a "we" to speak of.
The Gregorian year differs from the tropical year
by 0.000310 days, or equivalently by one day in
3226 years. One could propose, although this has
not been done, to add a leap day every 3000 years,
say in the years 3000, 6000, 9000 and so on. Of
course some of these are leap years already, meaning there would be one year every 6000 years with a
February 30th! (Think how few birthdays someone
born on that day would have!) This could easily be
prevented by simply moving the extra leap years to
off-cycle years, such as 3001, 6001 and so on. However, I think the most likely solution, should we ever
feel the need for one, would be to propose extra leap
days on an ad hoc basis, about one every 3000
years. This is essentially the way we in recent years
have imposed necessary leap seconds into the
clock.
Finally, precession is in no way eliminated by the
Gregorian calendar or even by any conceivable improvement to it. The equinoxes, and the perihelion,
still precess around the Sun due to the mismatch
between the sidereal and the tropical, or the sidereal
and the anomalistic, years. The equinox makes a
complete orbit of the Sun in approximately 26,000
years, and the North and South celestial poles make
circles 23.5 degrees in radius, forming fleeting (on
the time scale of millennia) alliances with stars they
happen to sweep nearby. The perihelion similarly
orbits the Sun every 112,000 years. (It is also proposed that precession of the perihelion and the equi-
noxes, as well as periodic shifts in both the eccentricity of the Earth's orbit and in its rotational inclination may play a role in long-term climate variation,
although the details are far from clear at this point.)
Since the seasons (and our modern calendar) are
determined by the Earth's position relative to the
equinoxes and not the equinoxes' absolute positions
around the Sun, over time different constellations will
be associated with different seasons. We in the
Northern Hemisphere associate Orion with the winter
and Scorpius with the summer, but in about 13,000
years, not only will Vega be the north polar star, the
constellations will appear in reverse seasons since
the summer and winter solstices will have traded
places around the Sun.
The gift offered by the astronomers to the more
"practical" sorts is a calendar that keeps phase with
the seasons; in the year 15,000 the Fourth of July
will not be celebrated in the dead of winter or some
other random season, but rather as always in the
heat of summer (possibly greatly enhanced by the
greenhouse effect, a phenomenon whose existence
was first inferred by studies of the planet Venus).
Our sacrifice will be that, providing that light pollution
has not already obliterated all memories of the night
sky, the sweltering evening's fireworks will be set
against a backdrop of Orion, Taurus, the Pleiades,
and the other stellar formations we of the present era
have come to associate with colder times.
Julian Day Number (continued from page 10)
by Dr. Lew Thomas
He needed to find an ancient date upon which all the cycles would start together and would produce the offsets
noted on 1 BC. That date became 4713 since
4713/28 = 158 with a remainder of 9 years
4713/19 = 248 with a remainder of 1 year
4713/15 = 314 with a remainder of 3 years
Starting the day count and the cycles on this date would cause the three cycles to separate by the above
amounts by 1 BC while at the same time produce JDNs which were all positive.
For example, the solar cycle is at 9 years at 1 BC. 9 years plus 158 cycles arrives at 4713 BC. (9 + 158x 28 =
4713).
And this is the reason that the JDNs start at noon on the Greenwich meridian on January 1, 4713BC. Of
course one has to pick a spot on the Earth from which to reckon time . The Greenwich meridian was agreed
upon somewhat after our Civil War.
14
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
GENERAL MEETING February 15, 2008
" A Ray of Light in
a Sea of Dark (Matter)" - Dr. Charles
Keeton, Rutgers
University
Dr. Charles Keeton
makes use of a sort
of natural “telescope”
— gravitational lensing — to study mysterious “dark matter”.
Dr. Keeton attributes
his interest in space
to the success of the
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Voyager missions and the Space Shuttle program in
the late 1970s and early 1980s. After earning a B.A.
from Cornell University and Ph.D. from Harvard University, Dr. Keeton did research at the University of
Arizona and the University of Chicago before joining
the faculty of Rutgers University in 2004. Dr. Keeton
has observed with the Hubble Space Telescope as
well as observatories in Arizona and Chile. His research has recently been featured by National Public
Radio, MSNBC.com and the New Scientist magazine.
8PM IN THE ROY SMITH THEATER
EMAIL CONTACTS
$21
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(Subscription renewals to S&T can be
done directly. See “Membrship-Dues”
on website for details.)
AAI Dues can be paid in person
to Membership Chair or Treasurer, or by mail to: AAI, PO Box
111, Garwood, NJ 07027-0111
DR. LEW’S SEMINARS
See Dr. Lew Thomas for possible
upcoming seminar topics.
(Choice of topic at Dr. Lew’s seminars
is determined by participants’ interest)
[email protected]
President of AAI
[email protected]
Editor of The Asterism
Ray Shapp, Editor
Deadline for submissions to each
month’s newsletter is the first Friday of
that month.
[email protected]
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[email protected]
Executive Committee plus
Trustees
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[email protected]
AAI president, corresp. secretary, and
computer services chair
DOME DUTY SCHEDULE
February 22
February 29
March 7
March 14
Team E
Team A
Team B
Team C
FRIDAYS AT SPERRY
February 22, 2008
Ask the Astronomers
Dr. Lew
February 29, 2008
The History of the Telescope Part 1 (Part of the
400th anniversary of the
telescope) Al Witzgall
March 7, 2008
What's Up: A Down-to-Earth Sky
Guide Kathie Vaccari
All schedules above were accurate at
time of publication. Please check
www.asterism.org for latest information (click on "Club Activities")
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
15
March 2008 is a quiet month for planet-watchers. A
few slow, lovely ballets, but nothing really spectacular.
tation occurs over the United States in midafternoon, the second over Australia. Two weeks
later, Uranus also flies by.
The leading couple would be Venus and Mercury.
Every year the two inner planets pass each other
half a dozen times or so. Usually these conjunctions
are quick affairs. A few cloudy days and we would
miss the whole thing. Not this month. These morning
"stars" will be less than three degrees apart for the
entire month, although fairly close to the Sun.
Very early risers can enjoy Jupiter, far to the upper
right of all the sunrise events. The southernmost
Moon of the month passes below the Giant Planet
near the beginning of the month, and then again
near the end.
This event really began around February
27th as Mercury, racing away from the
rising Sun, passed
about one degree
above Venus, which
has been slowly falling
toward the Sun since
the end of last October. But then, on the
3rd of this month, Mercury slams on the
brakes and also starts
slowly falling toward
the Sun. Now the two
planets fly in close
formation until Mercury catches up with
Venus on the 24th.
March Sky Calendar
2 Sun 5:30 AM Crescent Moon right of Jupiter
3 Mon 5:30 AM Crescent Moon lower left of Jupiter
3 Mon 6:00 AM Mercury at maximum elongation from the Sun
5 Wed 5:45 AM Venus, Mercury, and thin crescent Moon in tight formation
7 Fri 12:14 PM New Moon
8 Sat 6:45 PM Very thin crescent Moon directly above sunset point
9 Sun 2:00 AM Daylight Saving Time begins; turn clocks forward one
hour
14 Fri 6:45 AM First Quarter Moon
14 Fri 11:00 PM Mars just below Moon
18 Tue 8:30 PM Saturn-Regulus-Moon in short, straight line
20 Thu 1:48 AM Vernal Equinox; earliest start to spring since 1896
21 Fri 2:39 PM Full Moon
24 Mon 6:30 AM Mercury one degree below Venus
29 Sat 5:48 PM Last Quarter Moon
30 Sun 6:00 AM Moon lower right of Jupiter
31 Mon 6:00 AM Moon lower left of Jupiter
By that time the planets are probably too
low in morning twilight
to be observed. But if
you should find yourself up and about before sunrise
on any clear morning before the last week of March,
look to the southeast for Venus. If you spot this brilliant planet, grab your binoculars and scan a degree
or two to the right for Mercury, which is near zero
magnitude or brighter all month.
Around the beginning of this event, invisible Neptune passes less than one degree above the two
inner planets. The thin crescent Moon joins the
party on the 5th. Later that day the Moon passes directly over Venus and then Neptune. The first occul-
16
Prime time planet-watchers will have to content
themselves with Mars and Saturn. The Red Planet
is still high and bright, but no longer negative magnitude. Look off to the left of Mars for tiny Mebsuta, in
Gemini. Mars slowly moves toward the star all
month, passing a quarter of a degree below it on the
30th.
Saturn is up most of the night setting just before morning twilight. All month the Ringed Planet moves to the
right toward Regulus, the heart of Leo, the Lion.
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
Stunning Beauties of Our Solar System
Ken Kremer
New Territory from MESSENGER Mercury Flyby
NASA’s MESSENGER spacecraft successfully flew past the planet Mercury on 14 January 2008. The closest
approach was 124 miles altitude. After two more flybys in October 2008 and September 2009, the probe will
finally orbit the planet in 2011 after a journey of 5 billion miles through space. The orbit during the planned one
year science mission will be highly elliptical ranging from 125 miles to 9,420 miles. MESSENGER will orbit
Mercury twice every 24 hours. The probe was launched on a Delta II Heavy booster rocket on August 3, 2004
from Cape Canaveral Air Force Station. This false-color mosaic view was taken at an average distance of
about 9,000 miles and it shows a portion of the previously unseen and unknown side of Mercury. The large
circular light-colored area in the upper right of the image is the interior of the Caloris basin (see close-up next
page). Only about 45 percent of Mercury’s surface had been previously mapped by Mariner 10, 33 years ago.
MESSENGER was designed and built by John Hopkins University’s Applied Physics Laboratory (JHU/APL).
Image link: http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Prockter07.jpg
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Learn more at the MESSENGER Homepage: http://messenger.jhuapl.edu/index.php
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008
17
Science Outreach and Update
Image link: http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=2&gallery_id=2&image_id=144
by Ken Kremer
Spidery Crater on Mercury
MESSENGER spotted these highly
unusual rays emanating from a 25 mile
wide crater near the center of the giant
Caloris Basin. The feature was nicknamed “the spider” by the science
team. Nowhere else in the Solar System has this type feature been seen by
Earth’s robotic or human explorers.
The Narrow Angle Camera of the
Mercury Dual Imaging System
(MDIS) on the MESSENGER spacecraft obtained high-resolution images
of the floor of the Caloris basin on
January 14, 2008. This area had not
been photographed by Mariner 10, the
only spacecraft which previously visited Mercury during three fly-bys in
1974 and 1975.
Science Outreach at Riverside Elementary School: Princeton, NJ, Dec 6. At this annual astronomy
night, all of the 3rd grade students and their families enjoyed my presentations and displays on “Twin Robots
Exploring Mars”, shortly before Mars at Opposition. This year we were relocated to the more comfortable environs of the library with a newly installed projector. Thanks to rare clear weather, outdoor telescopes enabled
viewing of nebulas and even a few meteor streaks.
Left: Princeton area families “On Mars in 3-D” as Spirit and Opportunity celebrate 4 years of roving on the Red planet.
Right: Kids and kin gawk at Martian Craters and Saturn’s moons in 3-D at Riverside Elementary School on Dec 6
Please contact me for
further information or
science
outreach
presentations. My
upcoming
talks include:
Mill Lake Elementary School: Monroe Twp, NJ, Wed, Feb 13, 6:30 PM. “Exploring Mars at Astronomy Night”.
Website: http://monroenj.schoolwires.com/8602011112104530/site/default.asp
Note: This event will also feature AAI sponsored telescope viewing
Rittenhouse Astronomical Society (RAS) at the Franklin Institute: Philadelphia, PA, Wed, Feb 20, 8 PM.
“Lunar, Solar and Martian Eclipses”. http://www.rittenhouseastronomicalsociety.org
Raritan Valley Community College Planetarium: Somerville, NJ, Wed, Apr 2, 7:30 PM. “Launching DAWN
(and Phoenix): From Behind the Scenes at Kennedy Space Center”. http://www.raritanval.edu/planetarium
Washington Crossing Nature Center: Titusville, NJ, April 12, 1 PM. “Mars, Saturn, Asteroids and Beyond”
Dr. Ken Kremer
18
Email: [email protected] NASA JPL Solar System Ambassador
Asterism, the newsletter of Amateur Astronomers, Inc. February 2008