Download Planets and Transits

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A new view of the Universe III
Fred Watson
April 2005
What is a transit?
Transit geometry
Why isn’t there a Venus transit every year or so?
Venus’ orbit is tilted relative to Earth’s…
This makes transits of Venus
jolly rare events…
Cyclic phenomenon…
Transits of Venus
follow a 243-year
repeating pattern.
They always occur in
June or December
because that is when
the Earth crosses the
line of nodes.
The dawn of the telescope era…
Della Porta’s sketch
of 1609 shows the
principle.
But it was Galileo
who turned it into an
engine of discovery
in the same year
By the way, while
we’re talking about
the birth of the
telescope…
To be published in
the US September
by Da Capo
Put it on your
Christmas list
TODAY!
The 1639 transit
Drat!
I’ve missed
the good
bit.
Predicted by Jeremiah Horrocks (1618-1641).
Observed by him at Much Hoole, 4 Dec. 1639.
Had to dash off to church before the event started.
Venus was in mid-transit when he returned.
What he actually said…
‘Then I beheld a most agreeable
spectacle… a spot of unusual
magnitude and of a perfectly
circular shape…’
The 1639 transit
It was also observed by his friend William Crabtree
(1610-1644), a Manchester cloth-merchant.
More transit geometry
Why is a Venus transit potentially so useful?
It allows you to measure the distance to the Sun
Bright idea courtesy of
Edmond Halley (1656-1742)
The 1769 transit
By then, telescopes had improved significantly.
And governments
were prepared to
fund expeditions to
far-flung places to
get the best view of
the transit.
The 1769 transit
James Cook in Tahiti
(And the rest is history…)
The trouble with transits
Cook and others
found their timings
spoiled by the blackdrop effect...
1874
The 1874 transit in Australia
Henry C. Russell:
‘Never perhaps in
the world’s history
did morning dawn
on so many waiting
astronomers as it did
on 9 Dec. 1874.’
He observed from
Sydney, Woodford,
Eden and Goulburn.
Russell’s rival, John
Tebbutt, disparagingly
described his report on
the 1874 transit as a
‘…gorgeous volume…’
And again in 1882…
Consternation!
German transit expedition in Chile, 1882
Photographic assistance
Taken by
William
Harkness of
the US Naval
Observatory,
6 Dec 1882.
(It was cloudy
in Sydney.)
Harkness looks to the future…
‘We are now on the eve of the second
transit of a pair, after which there will
be no other till the twenty-first
century of our era has dawned upon the
earth, and the June flowers are
blooming in 2004…
What will be the state of science when
the next transit season arrives, God
only knows.’
(Dec 1882.)
The transit of June 2012…
Don’t look directly at the Sun (especially not
through binoculars or a telescope)!
Use eyepiece projection (like Horrocks)
Or check webcasts from NASA and other
agencies (unlike Horrocks).
But what actually is a planet…?
Isn’t it obvious?
It’s one of these…
Or these…
In fact, it’s a
serious
contemporary
problem.
Or maybe one of these…
Is it a star…?
What about ultra-low
mass stars? Brown
dwarf ~40 Jupiters
An object that shines only by reflected light?
An object that orbits the Sun…?
To detect Jupiter need
12.5 m/s accuracy.
For Saturn 2.7 m/s.
Attainable accuracy 2 m/s.
An object that orbits the Sun…?
Gliese 876
An object that orbits a
star…?
Free-floating objects
in Orion with masses
less than 13 Jupiters
An object big enough for its own gravity to
crush it into a spherical shape?
951 Gaspra
And what about Pluto compared with Kuiper
belt objects?
The astronomical community wrestles with
the definition of a planet…
How are planets distinct from moons, asteroids,
brown dwarfs, stars?
Response of the International Astronomical Union
(the only body that can make the definition) is:
‘Well, er…’
Or, to be more precise:
1) Objects orbiting around solar-type stars with true masses above the limiting mass for
thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for
objects of solar metallicity) are "brown dwarfs" (no matter how they formed) while
objects with true masses below this limiting mass are "planets".
2) Free-floating objects in young star clusters (which presumably formed in the same
manner as stars and have not been shown to be ejected from planetary systems) with
masses below the limiting mass for thermonuclear fusion of deuterium are not "planets",
but are "sub-brown dwarfs" (or whatever name is most appropriate).
What is the use of transits today?
So, looking for the characteristic dimming is a
good way to find extra-solar planets…
And it might tell us much about the the planet
and star, as well as how many are out there…
Hence new telescopes…
…such as the 0.9-m
RCT (Robotically
Controlled Telescope)
at Kitt Peak in Arizona.
Jeremiah Horrocks
would have thought
this was a pretty
neat way to see
planetary transits...