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Telling Time by the Sun
Martha P. Haynes
Goldwin Smith Professor of
Astronomy
Cornell University
CAU Study Tour
June 2014
The Arecibo Legacy Fast ALFA (ALFALFA) Survey
http://stefanoderosa.com
It is aisradio
ALFA
not “camera”
a car…
Arecibo L-band Feed Array (ALFA)!
The ALFALFA Census of GasBearing Galaxies at z ~ 0
Arecibo
Legacy Fast
ALFA
(ALFALFA)
Survey
The sky here this week
Day
sunrise
sunset
moonrise
moonset
moonphase
Mon
16
05h32m
21h07m
23h38m
09h24
Waning gibbous
0.85 full
Tue
17
05h32m
21h07m
00h14m*
10h38
Waning gibbous
0.75 full
Wed
18
05h32m
21h07m
00h46m*
11h51m
Waning gibbous
0.65 full
Thu
19
05h29m
21h02m
01h14m*
12h57m
Last quarter
Fri
20
05h30m
21h03m
01h45m*
14h06m
Waning crescent
0.43
Sat
21
05h30m
21h03m
02h18m*
15h14m
Waning crescent
0.32
http://aa.usno.navy.mil/data/docs/RS_OneDay.php
Parma: 10°20’ E, 44°48’ N
Bologna: 11°21’ E, 44°30’
* On following day
Summer solstice
Sat June 21 @
10h51m UT =
12h51m CEST
For later in the week:
Bologna on Saturday
http://aa.usno.navy.mil/data/docs/RS_OneDay.php
Bologna: 11°21’ E, 44°30’
Summer solstice
Sat June 21 @ 10h51m UT = 12h51m CEST
What the ancients observed.
1. The Earth is spherical.
2. The Sun rises in the East and sets in the West every ~24 hours
(diurnal westward motion).
3. Over the course of a year, the position of the Sun drifts slowly
eastward among the stars and returns to the same place among the
stars every ~365 days (annual eastward drift).
4. The Sun, Moon and planets are confined to a band stretching
around the sky (ecliptic = zodiac).
5. The phases of the Moon; times of Moon Rise, Moon set.
6. Eclipses of the Sun and of the Moon
7. Differences in the motions of the planets.
What we already know that the ancients didn’t.
1. The Earth rotates on its axis once every ~24 hours (day).
2. The Earth revolves around the Sun once every ~365 days
(year).
3. The Moon revolves around the Earth once every ~28 days
(month).
4. The orbital planes of objects in the Solar System lie (almost) in
the equatorial planes of the major body (but not quite; there
are a few exceptions)
5. The Earth’s spin axis is tilted by 23½° with respect to its
orbital plane around the Sun (ecliptic).
The Constellations
Stars that appear close in the sky may not actually be close in space:
The Earth’s Orbital Motion
• The path of the Sun’s motion through the constellations of stars is
called the ecliptic.
• The 12 constellations that Sun moves through during the year are
called the constellations of the zodiac.
The Earth’s orbital motion
• Seasonal changes in the constellations visible in the night sky are
due to Earth’s motion around Sun
Celestial Sphere
Celestial sphere:
Imaginary sphere of huge
radius centered on Earth and
aligned with Earth’s poles.
The concept of the celestial
sphere provides a convenient
visual framework for
understanding the apparent
motions of the Sun, Moon,
planets and stars.
“Coordinates”:
Right ascension (like longitude)
Declination (like latitude)
Three Perspectives of the Sky
Heliocentric
Geocentric
Local
Centered on Sun with
Sun fixed
Centered on Earth
with
Earth fixed
(not moving)
Centered on observer
(Your view of the sky)
Celestial sphere:
Imaginary sphere of huge
radius centered on Earth and
aligned with Earth’s poles.
Our view of the celestial sphere
depends on our location on Earth.
Local Perspective
Zenith
Local
meridian
Follow
along with
the
practice
page
handout
Local west
Local
south
Local
north
Local east
Horizon
The azimuth is the angular distance along the
horizon from the north point measured in the
eastward direction.
Local Perspective
Zenith
Local
meridian
W
Az=
180º
Az= 270º
S
N
E Az= 90º
Az=
0º
The azimuth is the angular distance along the
horizon from the north point measured in the
eastward direction.
Local Perspective
Zenith
Local
meridian
NCP:
North
Celestial
Pole
W
altitude
S
N
E
Horizon
The altitude of the North Celestial pole (as
measured up from the horizon)
is equal to the latitude of the observer’s location
Local Perspective
C.Eq.
NCP:
W
S
altitude
N
The celestial equator crosses the meridian
90° to the south (measured along the
meridian) of the NCP.
Local Perspective
C.Eq.
NCP
W
S
altitude
The altitude of point where the celestial equator crosses
the local meridian is 180º - altitude of NCP - 90º =
altitude of C.Eq. = 90º - altitude of NCP
N
Diurnal Motion
• As the Earth rotates, a star (the Sun, the Moon or a planet),
appears to move across the sky, from East to West, following a
path parallel to the Celestial Equator.
Along Lines of Constant
Diurnal
East to
Declination, parallel to
Motion
West
the Celestial Equator
• Since the orientation of the lines of Declination depend on the
location (latitude) on Earth, the path a star (the Sun, the Moon
or a planet) takes across the sky depends on the latitude of
the observer’s location on Earth.
The zodiac = the ecliptic
During the year, the Sun appears to move through the
constellations of the Zodiac.
In astronomical lingo, the zodiac is the “ecliptic”.
The ecliptic: the annual path of the Sun
Due to the Earth’s orbital
revolution, the apparent
position of the Sun in the
sky appears to move along
a path tilted by 23½ deg
with respect to the
celestial Equator (higher
up North in northern
Summer, higher up South
in southern Summer. The
apparent path of the Sun
amid the stars in the sky
is referred to as the
“ecliptic”.
The Sun’s Apparent Path
• The Sun’s apparent position among the stars changes throughout
the year.  Eastward annual drift
• Unlike a star, the Sun (Moon and planets) moves with
respect to the (much more distant) stars.
• Right Ascension and Declination of the Sun (Moon and
planets) change throughout the year.
• The path the Sun (Moon and planets) takes across the sky on any
given day depends on its Declination on that day.
• Noon-time altitude (above horizon) varies
• Length of time to cross from East to West along the
path on a given day varies = length of daylight.
The Sun’s Apparent Path
• The Sun’s apparent position among the stars changes throughout
the year.  Eastward annual drift
• Unlike a star, the Sun (Moon and planets) moves with
respect to the (much more distant) stars.
• Right Ascension and Declination of the Sun (Moon and
planets) change throughout the year.
• The path the Sun (Moon and planets) takes across the sky on any
given day depends on its Declination on that day.
• Noon-time altitude (above horizon) varies
• Length of time to cross from East to West along the
path on a given day varies = length of daylight.
The Earth experiences seasons!
Tilt of the Earth on its axis
The Ecliptic Plane
The Sun’s Path Throughout the Year
• The Sun’s Declination changes throughout the year due to the
inclination of the Earth on its axis.
• On Sep 20th and Mar 20th, the Sun’s Declination is 0°.
• The Sun’s path follows the Celestial Equator.
• These are called the autumnal and vernal equinoxes.
• On Dec 21st, the Sun’s Declination is -23½°.
• At noon, the Sun crosses the meridian south of the Celestial
Equator by 23½°.
• Length of daylight shortest in North; longest in South.
• Winter in the northern hemisphere; summer in the South.
• On Jun 21st, the Sun’s Declination is +23½°.
• At noon, the Sun crosses the meridian north of the Celestial
Equator by 23½°.
• Length of daylight longest in North; shortest in South
• Summer in the northern hemisphere; winter in the South.
The Sun’s Path Throughout the Year
Facing south
The Sundial in
Busseto
• Busseto is a commune in
the province of Parma with
a population of ~ 7100
• Rocca built in 1st half of
13C; destroyed and rebuilt
in 1857.
• Giuseppe Verdi was born in
Roncole in 1813 and moved
to “town” (Busseto) in
1824.
Basic scheme of a vertical sundial
The Sundial in Busseto
A not quite south-facing sundial
Zodiac symbols
The sky as seen from Busseto
The sky as seen from Busseto
The sundial in
Busseto
The Sun today in Busseto
Busseto 44°59’ N, 10°02’ E
(44.98333,10.03333)
The Sun today in Busseto
Busseto 44°59’ N, 10°02’ E
(44.98333,10.03333)
South-facing vertical sundial
E
W
S
South-facing vertical sundial
E
W
S
Sundial in Busseto