Lab 1: The Celestial Sphere and the Seasons
Name: _________________________________
Lab Partner(s): _________________________________
Section: _________________________________
Purpose: In this lab, you will become familiar with the altitude-azimuth and the right ascensiondeclination coordinate systems, and will learn why we have seasons on Earth.
PART I: Celestial Sphere
Farquhar Globe: To complete this lab, you’ll be using a Farquhar globe.
1. The outer globe represents the celestial sphere and the inner globe represents the Earth. Along
the celestial sphere are various constellations, each with its own boundary shown by blue
lines. The small yellow ball represents the Sun, and the movable ring around the Earth
represents the horizon.
2. The point where the rod holding the Earth hits the bottom of the outer globe is the south
celestial pole. Opposite to this is the northern celestial pole. These are simply extensions of
the poles of the Earth.
3. The place where the two outer hemispheres meet is known as the celestial equator. This is
simply an extension of the Earth’s equator.
4. You can rotate the Earth in the clockwise direction as viewed from outside the globe by
turning the knob at the bottom of the globe. Please don’t turn it counterclockwise, since that
will disassemble the globe. One full clockwise turn represents one day. (If you wished to, you
could hold the Earth still and turn the celestial sphere. Hence, the daily rotation of the Earth
and the apparent rotation of the stars around the Earth are equivalent for this lab.)
5. You can make the Sun appear to revolve around the Earth by turning the knob close to the
north celestial pole. One revolution of the Sun represents one year. We all know that the Earth
really revolves around the Sun, but for this lab it’s easier to think of it the “wrong way.”
6. The Sun has a very specific path along the celestial sphere. This is called the ecliptic. The
ecliptic corresponds to the plane defined by Earth’s orbit around the Sun. Notice how the
ecliptic on our Farquhar globe is marked with the dates. These indicate when the Sun appears
to lie at a given point on the celestial sphere.
Examining the Celestial Sphere
Instructor Example: At the summer solstice, what constellation is the sun in?
1. Approximately 1100 stars are represented on the sphere. Yellow dots of seven sizes represent
stars of different brightness. What is the name of the brightest star in the constellation Gemini?
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2. On what two dates does the Sun, moving along the ecliptic, cross the celestial equator?
These dates are called the equinoxes. The vernal equinox marks the beginning of spring, and the
autumnal equinox marks the beginning of fall.
3. On what dates is the Sun farthest from the celestial equator?
These dates are called the solstices. The summer solstice is the first day of summer, and the
winter solstice is the first day of winter.
4. At the vernal (spring) equinox, what constellation is the sun in?
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5. The Earth's rotational axis is not perpendicular to the ecliptic plane; instead, it is tilted by
23.5. As the Earth revolves around the Sun, its tilt causes the seasons. Think about the
physical meanings of the equinoxes and solstices, and how they are caused by the Earth's tilt.
On which of these days would the Sun appear highest in the sky? ______________________
Lowest? _____________________________ You will examine this more closely later in the
lab.
Right Ascension and Declination
Right ascension (RA) – Like longitude; it is measured along the celestial equator in hours and
minutes. Each hour is equivalent to 15, and each hour is made up of 60
minutes.
Declination (Dec) – Like latitude; it is measured in degrees northward and southward from the
Instructor Example: What is the RA and Dec of the Sun at the winter solstice?
Complete the following table:
Object
Vernal Equinox
Summer Solstice
Autumnal Equinox
Sirius
RA
Dec
0h 0min
0°
celestial
Altitude, Azimuth, and Other Observer-Dependent Terms
Set up the Farquhar globe to measure altitude and azimuth for a given location at a given time.
1. Move the Sun to the position that corresponds to the desired day.
2. Put the location (ex. College Park) at the top of the world!
3. Move the horizon ring until it is parallel with the floor. The labels on the ring should be facing
up, and the gap in the ring should be towards you.
4. Hold onto the knob on the bottom so that the Earth doesn’t move. Rotate the celestial sphere
until the sun is at the desired position. For example, at noontime, the sun is as directly
overhead as possible – it is on your meridian. Do NOT move the sun with respect to the
constellations, because this would be changing the day. We simply want to change the hour.
Zenith – The point on the celestial sphere that is directly overhead.
Meridian – Imaginary line on the celestial sphere that goes from the north point on the horizon,
through the north celestial pole, and through our zenith to the south point on the
horizon.
In other words, an imaginary line that splits the eastern half of the sky from the
western
half.
Altitude – Angular distance measured vertically above or below the horizon to a given object.
The zenith has an altitude of 90, while the horizon has an altitude of 0. Altitudes
below the horizon are negative.
Azimuth – The “compass direction” toward an object measured eastward around the horizon
from north. The direction N has azimuth 0, NE has azimuth 45, E has azimuth 90,
S has azimuth 180, W has azimuth 270, etc.
Circumpolar Constellation – Constellations which are always above the horizon; they never set.
Instructor Examples: Demonstrate how to determine altitude and azimuth, and find a
circumpolar constellation for College Park.
Set your globe up for Paris, France at noon on June 10th, and fill in the following table.
Date
June 10
Time
Noon
Location
Longitude
Latitude
Sun's RA
Dec
Altitude
Azimuth
Paris, France
2.3° E
48.8° N
While you’re in Paris, find two circumpolar constellations. ______________________________
Putting it Together: Why the Seasons?
The Earth's 23.5 degree tilt causes the Sun's position in the sky to change throughout the year,
leading to the seasons. In this part of the lab, you will simulate the passing of a year on your
celestial sphere to see exactly how the seasons come about.
Winter Solstice: Sun is 23.5 degrees below the Celestial Equator and is directly overhead the
Tropic of Capricorn at noon.
1. Position the globe so that College Park is on top. Move the plastic horizon marker so it's
horizontal. Set the date to December 22, at noon.
2. Imagine you are standing on the globe, in College Park. Would you have to look north or
south to see it? _______________ Measure its altitude: __________ It would appear fairly
low in the sky, even at noon, when it is highest.
3. What is the right ascension of the meridian at noon?____________ Move the globe so that it's
sunset (Sun is on the western horizon). What is the RA of the meridian now?_____________
How long did it take the Sun to set (subtract the two RA's)? _____________________
4. How long would daylight last in College Park on Dec. 22 (should be twice as long as the time
from noon to sunset)? ____________________
Vernal Equinox: Sun crosses the Celestial Equator and is directly overhead the Equator at noon.
5. Now set the date to March 22 at noon. Would you look north or south to see the Sun? ______
Measure its altitude: ___________ Would it appear higher or lower in the sky than on the
winter solstice? ________________________________________________________
6. What is the right ascension of the meridian at noon? ___________ Move the globe so that it's
sunset. What is the RA of the meridian now? ______________ How long did it take the Sun
to set? ____________________
7. How long would daylight last in College Park on March 22? ______________________
Summer Solstice: Sun is 23.5 degrees above the Celestial Equator and is directly overhead the
Tropic of Cancer at noon.
8. Set the date to June 21 at noon. Would you look north or south to see the Sun? __________
Measure its altitude: _____________ Will it appear higher or lower in the sky than on the
vernal equinox? _____________________________________________________________
9. Note the RA of the meridian. Change the time to sunset and note the RA of the meridian
again. How long did it take the Sun to set? ______________________
10.
How long would daylight last in College Park on June 21?
_________________________
Autumnal Equinox: Sun crosses Celestial Equator.
11.
The Sun will have the same altitude, and the length of daylight will be the same,
as on the vernal equinox. No need to repeat all those measurements. But take note: the Sun
has been getting lower in the sky ever since the summer solstice. On the autumnal equinox, it
crosses the celestial equator, and continues to get lower in the sky until reaching the winter
solstice again.
Questions:
12.
How does the Sun's altitude change throughout the year? Give a conceptual
answer – no numbers for altitudes are necessary.
13.
How does the altitude correspond to the length of daylight?
14.
How will the Sun's altitude affect how much the sunlight is concentrated in
College Park?
15.
What is the underlying reason that the Sun's altitude changes at all, giving us
seasons? (Hint: It's a characteristic of the Earth.)
16.
Explain why we have seasons on Earth, using your answers to questions 12-15
to guide you.