The View From Earth
Outline
1. Earth in Motion
A. Rotation: 24 hrs
a. CCW about north pole
b. 750 mi/hr at State College
c. Night/day
B. Revolution: 365.25 days
a. CCW about north pole
b. Avg. speed: 67,000 mi/hr
C. Orientation of Earth in Space
a. 23.5o tilt of polar (rotational) axis
D. Precession: 26,000 yrs
a. ‘Wobble’ of Earth’s polar axis
b. Due to gravity of Moon and Sun
E. Orbit about center of Milky Way: 230 million yrs
a. Orbit radius: 28,000 ly
b. Speed: 500,000 mi/hr
F. Motion within Local Group
a. Galaxies ‘swarm’ due to mutual gravitational attraction
b. Collion with M31 (?)
G. Expansion of the University
a. Space itself expanding ( ‘raisin-cake’ model)
2. Latitude, Longitude & All That
A.
B.
C.
D.
Latitude: angle between equator plane & your location (0o < lat < 90o)
Latitudes of tropic & polar circles related to Earth’s orientation
Longitude: measures E/W of Prime Meridian (0o < long < 180o E/W)
State College: lat = 41o N; long = 78o W
3. Celestial Sphere: A Model of the Sky
A. Horizon (plane), Meridian, Zenith
B. Celestial poles (Polaris); Celestial Equator
C. Rising & Setting
a. Due to Earth’s rotation
b. Stars follow circles centered about celestial pole
c. Circumpolar stars
4. The Sky & Your Latitude
A. Altitude of celestial pole = your latitude
B. Daily motion of sky at various latitudes
C. Drift of celestial poles due to precession
5. Different Stars in Different Seasons
A. Due to orbit of Earth about Sun
6. Constellations
A. Traditional and Modern definitions
B. A few star names
Diagrams
Questions
1. Ques. #11, pg. 26.
2. All points on Earth rotate with the same period, namely 24 hrs. So, why does a point on Earth's
equator rotate about Earth's axis with a speed greater than the rotation speed of State College?
3. Ques. #2, pg. 48.
4. Ques. #4, pg. 48.
5. Ques. #6, pg. 48.
6. Ques. #7, pg. 48.
7. Prob. # 2, pg. 49. [Hint: As reported in class, for State College: Lat = 41o N; Long = 78o W.]
8. Suppose the angle between Earth's equatorial and ecliptic (orbit) planes were only 16.5o. What
would be the latitude of the Tropic of Cancer and the Arctic Circle?
9. Your spaceship lands on the mysterious planet Xenon at a locale where all stars appear to circle a
point elevated 57o above your northern horizon. What's your latitude on Xenon? [Hint: Assume
Xenonians measure latitude in exactly the same way we do; i.e., equator lies at 0o lat & north pole lies
at 90o lat]
10. As observed from State College, a star that rises directly East must set where on the horizon? A
star that sets in the southwest must have risen where?
11. [Multiple-Choice] A certain star appears on your zenith at midnight on a certain night. About how
much time must pass before this same star appears again at your zenith?
a)
b)
c)
d)
One hour
One day
One month
One year
12. Where on Earth does the celestial equator coincide with the horizon?
13. Where would you be standing on Earth if no visible stars are circumpolar?
14. Take a look at Figure 2.13 (pg. 34) in your textbook. (a) The center of all of the arcs of circles
traced by stars in this picture is what special point on the sky? (b) Is the star Polaris found in this
picture? Explain. (c) How would this picture change if the viewer were standing at Earth's south
pole, instead of in Utah? (Apart from the obvious change in topography!)
15. The constellation Pegasus appears directly overhead at midnight. One month later, at midnight,
will Pegasus be found west of your meridian, or east of your meridian? Explain.
16. Precession has what effect on the appearance of the sky?
17. Ques. #1, pg. 48.
Answers
1. Earth: (1) rotates on an axis once every 24 hours; (2) revolves about the Sun with period 365.25
days; (3) accompanies the Sun (and other planets) as it moves relative to other stars in its immediate
neighborhood; (4) orbits about the center of the Milky Way galaxy, with period 230 million years; (5)
moves with the Milky Way galaxy as the Milky Way moves relative to other galaxies within the Local
Group; and (6) partakes, with the Local group as a whole, in the expansion of the universe.
2. Each point on the equator is carried through a larger circle than each point at the latitude of State
College. Consequently, points at the equator must be going faster as they must cover a greater distance
in the same amount of time (namely, 24 hours).
3. The celestial sphere is an imaginary sphere centered on any observer of the sky on Earth. The
celestial sphere is essentially infinite in radius. The sphere is divided into a visible part and an
invisible part by the horizon plane; the edge of the horizon plane forms the observer’s horizon. The
point directly overhead on the sphere is called the zenith. A half circle running from north to south,
and through the zenith, which divides the visible part of the sphere into an east half and a west half is
called the celestial meridian. Observation shows that all objects visible on the sphere (stars, planets,
Sun, Moon, etc.) appear to pivot every 24 hours about a point elevated above the north point on the
horizon (for a northern hemisphere observer). This point is called the celestial north pole. (A similar
point exists in the sky visible from Earth’s southern hemisphere.) The poles of the sky are located on
the celestial sphere above the north and south poles of Earth. A circle, called the celestial equator,
divides the sky into north and south halves; the equator is thus located midway between the north and
south celestial poles.
4. The sky looks like a dome because all stars appear approximately equidistant from you – although
in fact you have no idea as to their true distances. Horizon, zenith and (celestial) meridian are
mentioned in the answer to Ques. #3.
5. As your latitude changes, the altitude (elevation) of the celestial poles changes. For a northern
hemisphere observer, this means that the altitude of polaris (the “north star”) changes. Also,
constellations visible above the southern horizon change, as well. For example, were you to travel
south from State College (say, toward Florida), you would find that polaris’ altitude decreases, fewer
constellations are circumpolar, and new constellations, not previously visible, appear above your
southern horizon.
6. As Earth circles the Sun in its orbit, the night-time hemisphere (which always points away from the
Sun) naturally changes the direction in which it points. For example: the night-time hemisphere
points in opposite directions on July 1 and Jan 1. Here’s a diagram that illustrates this situation:
July 1
Sun
Jan 1
7. b) As viewed from State College, the north celestial pole (which is nearly coincident with Polaris)
is elevated about 41o above the north point on the horizon.
c) Yes, Polaris is circumpolar. Polaris circles the pole once every 24 hours, never touching the
horizon.
d) The (celestial) meridian sprouts from the north point on the horizon, passes through your zenith,
then disappears at the south point.
e) The celestial equator arises from the east point on your horizon, arcs upward to the celestial
meridian, crossing the meridian south of the zenith, then arcs downward to the west point on the
horizon.
8. Tropic of Cancer: 16.5o North; Arctic Circle: 73.5o N
9. Your latitude is 57o N.
10. A star that rises directly east must set directly west. A star that is observed to set in the southwest
must have risen in the southeast.
11. b) One day.
12. The celestial pole coincides with the horizon at either pole.
13. If no visible stars are circumpolar, you would be standing on Earth’s equator.
14. a) The center of the arcs is the north celestial pole (the picture was taken in the northern
hemisphere).
b) Polaris is the bright arc near the center.
c) At Earth’s south pole, the center of the arcs would be elevated 90o above the horizon; i.e., it
would appear in your zenith.
15. Pegasus will appear west of your meridian one month later. This shift is due to Earth’s revolution
about the Sun.
16. Among other effects, precession causes the location of the celestial poles to wander relative to the
stars. At the moment, the north celestial pole happens to lie near a moderately bright star (Polaris). In
the future, the pole will wander away from Polaris, and appear near Polaris again in about 26,000
years.
17. In modern terms, a constellation is a specific, well-defined region of the celestial sphere – rather
like a state on a map of the U.S. Traditionally, a constellation is the depiction of a person, animal or
object among the stars. Each modern constellations have been defined so as to enclose a traditional
constellation.