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 ascension-declination 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.
  1. 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.
  1. The place where the two outer hemispheres meet is known as the celestial equator. This is simply an extension of the Earth’s equator.
  1. 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.)
  1. 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.”
  1. 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?

______

  1. 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?

______

  1. 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 celestial equator.

Instructor Example: What is the RA and Dec of the Sun at the winter solstice?

Complete the following table:

Object / RA / Dec
Vernal Equinox / 0h 0min / 0°
Summer Solstice
Autumnal Equinox
Sirius

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.
  1. Put the location (ex. College Park) at the top of the world!
  1. 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.
  1. 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 / Paris, France
Longitude / 2.3° E
Latitude / 48.8° N
Sun's RA
Dec
Altitude
Azimuth

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.