Download The Celestial Sphere

The Celestial Sphere
Name _________________________
The Celestial Sphere
Critical Thinking Questions
1)
Why does the solar day differ in length from the sidereal day? Use a sketch to help
explain your answer.
The sidereal day is measured by Earth’s rotation on its own axis against the
background stars. Earth completes one rotation against the background stars in
23h 56m. As a consequence, stars rise about 4 minutes earlier each day producing
a slow drift of the constellations across the sky during the year.
The solar day is measured by Earth’s rotation on its own axis against the Sun.
Earth completes one rotation against the Sun in 24h 00m.
Figure 3.1 illustrates that a star aligned with the Sun on one day is not aligned
with the Sun on a subsequent day since Earth moves in its orbit. Figure 3.2
illustrates this same idea by showing the Sun’s motion along the ecliptic.
Figure 3.1
Figure 3.2
2)
Why does the Sun not appear in the correct zodiac constellation on your birthday?
Earth wobbles on its axis like a spinning top as it slows down. This is called
precession and it changes the location of the vernal equinox over time.
1
The Celestial Sphere
3)
Name _________________________
Why are all of the planets aligned so close to the ecliptic?
The planets all sit near the ecliptic because the Solar System is flat. The planets
orbit near the Sun’s equatorial plane with very little movement north to south.
Calculations
1) How high and above what horizon is the Sun on the given date at the specified
latitude? Assume that the time is local noon on the indicated date.
a) June 21 at the North Pole
23.5° above south horizon
b) December 21 at the North Pole
23.5° below south horizon
c) March 21 at the equator
90° (zenith)
d) June 21 at the equator
66.5° above north horizon
e) December 21 at the equator
66.5° above south horizon
f) June 21 in San Antonio (29.5° N)
84° above south horizon
g) December 21 in San Antonio (29.5° N)
37° above south horizon
2
Motions of the Sun
Name _________________________
Motions of the Sun
Important Facts
1)
a) What is Earth’s rate of rotation on its axis?
b) What is Earth’s rate of revolution about the Sun?
2)
On approximately what date do these astronomical events occur in the northern
hemisphere?
a) Vernal equinox
March 21
b) Summer solstice
June 21
c) Autumnal equinox
September 21
d) Winter solstice
December 21
3)
What is the single most important factor that determines why Earth (or almost any
planet in the Solar System) experiences seasons? Tilt of Earth’s axis
4)
On approximately what day of the year is Earth:
a) closest to the Sun?
January 4
b) furthest from the Sun?
July 4
15  hr
1  day
5) What two factors combine to drive temperatures higher in summer?
more direct sunlight and longer daytime hours
6) In what direction (Due E, Due W, NE, SE, NW, SW, Zenith) does the Sun appear to
rise or set on the following dates:
a) sunrise on March 21?
Due E
b) sunrise on June 21?
NE
c) sunrise on December 21?
SE
d) sunset on September 21?
Due W
e) sunset on June 21?
NW
f) sunset on December 21?
SW
Critical Thinking Questions
1)
The 2000 Olympics were held in Sydney, Australia in late September and early
October. This is about 6 – 8 weeks later than the normal July – August period when
the summer games are held. Why did the International Olympic Committee
schedule the 2000 games around the equinox rather than closer to the solstice?
3
Motions of the Sun
Name _________________________
Australia is located in the southern hemisphere, so they experience winter in July
and August. By holding the Olympics in late September and early October,
Australia had emerged from winter and was experiencing early spring.
2)
Is it possible for a planet with a perfectly circular orbit about the Sun to experience
seasons? Explain your answer.
Yes, planets in perfectly circular orbits will experience seasons if they are tilted on
their axes relative to the Sun. The tilt of the axis would still cause each hemisphere
to experience more or less sunlight at various times of the year.
3)
Mars’ orbit is substantially more eccentric than Earth’s orbit. Like Earth, Mars
reaches perihelion during northern winter. Describe the effect that Mars’
eccentricity has upon its seasons.
The northern hemisphere seasons on Mars would be significantly milder than the
summer hemisphere seasons. The southern hemisphere is closest to the Sun in
summer and farthest from the Sun in winter. Due to its higher eccentricity, the
difference in distance from the Sun will provide a measurable increase in
temperature in southern summer compared to northern summer, and a measurable
decrease in temperature in southern winter compared to northern winter.
4
Motions of the Moon
Name _________________________
Motions of the Moon
Important Facts
1)
Eclipses
a) Sketch the Sun-Earth-Moon alignment for a solar eclipse.
b) Sketch the Sun-Earth-Moon alignment for a lunar eclipse.
5
Motions of the Moon
2)
Name _________________________
Complete the following table of lunar phases:
Phase
Sketch
Moonrise/Moonset
Orientation to Sun
Moonrise: sunrise
New
Moonset: sunset
0°
Moonrise: after dawn
Waxing Crescent
Moonset: after sunset
< 90º east of the
Sun
Moonrise: noon
First Quarter
Moonset: midnight
Moonrise: Afternoon
Waxing Gibbous
Moonset: After midnight
90º east of the Sun
Between 90º to 180º
east of the Sun
Moonrise: sunset
Full
Moonset: sunrise
180°
Moonrise: after sunset
Waning Gibbous
Moonset: after dawn
Between 90º to 180º
west of the Sun
Moonrise: midnight
Last Quarter
Moonset: noon
90º west of the Sun
Moonrise: After midnight
Waning Crescent
Moonset: Afternoon
< 90º west of the
Sun
6
Motions of the Moon
Name _________________________
Critical Thinking Questions
1)
Why is the lunar month different from the sidereal month? Draw a sketch to
accompany your explanation.
It takes 27.3 days for the Moon to complete one orbit of Earth as measured against
the background stars. It takes 29.5 days for the Moon to complete 1 cycle of its
phases. The difference arises because Earth completes approximately 1/12 of its
orbit around the Sun in one month. As a result, the Moon needs an extra 2.2 days to
recreate the original Sun-Moon-Earth alignment from one New Moon to the next
New Moon.
Figure 4.1 illustrates this by showing that the Star-Moon- Earth alignment is not the
same each month as the Sun-Moon-Earth alignment. Figure 4.2 assumes a New
Moon at the vernal equinox. After 27.3 days, the Moon completes 1 trip along the
ecliptic, but the Sun has moved 1/12 of the way along the ecliptic during the same
time.
Figure 4.1
Figure 4.2
7
Motions of the Moon
2)
Name _________________________
Why are some solar eclipses total while others are annular? Were total eclipses or
annular eclipses more common in the distant past? Which will be more common in
the distant future? Explain your answers.
Some solar eclipses are annular and others are total because the Moon’s orbit is
elliptical. When the Moon eclipses the Sun at perigee (its closest point), it is just
slightly larger than the disk of the Sun. When the Moon eclipses the Sun at apogee
(its farthest point), it is just slightly smaller than the disk of the Sun.
Because the Moon is receding from Earth, it was closer to Earth in the past and
therefore it appeared larger. Therefore, total solar eclipses were more common than
annular solar eclipses in the distant past.
In the far future, the Moon will recede far enough from Earth that it will never again
completely cover the disk of the Sun. When that happens, observers from Earth will
only see annular solar eclipses.
3)
If the Moon is receding from Earth, then the angular momentum in its orbit is
increasing. Where does it get this additional angular momentum?
The additional angular momentum in the Moon’s orbit comes at the expense of
Earth’s rotation rate. Earth’s rotation is slowing down at the rate of approximately 1
second per century.
8