Lab 1: the Celestial Sphere

Lab 1: The Celestial Sphere and Planisphere

Name: ______

Lab Partner(s): ______

Section: ______

Purpose: In this lab, you will become familiar with the altitude-azimuth and the right ascension-declination coordinate systems. You will also learn how to use a planisphere.

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?

______

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 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:

Altitude, Azimuth, and Other Observer-Dependant 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.

While you’re in Paris, find two circumpolar constellations. ______

______

PART II: Planisphere

RA-Dec coordinates are only good for storing the location of stars in reference books. This coordinate system, called the equatorial system, tells us nothing about how to actually find something in the sky when we go outside. The altitude-azimuth coordinate system, called the horizon system, fulfills the opposite role: it tells us how to find something in the sky at a given time at a particular location. If somehow we could convert between these systems, we could look up the equatorial coordinates of the star in a reference book, and knowing the time and place of the observation, we could convert these coordinates to horizon coordinates, go outside, and find the star! The conversions, however, are quite complex and involve some rather complicated equations and techniques. Fortunately, a wonderful tool is available to us to make these conversions more easily: a planisphere.

The planisphere consists of two parts, the inner rotating wheel and the outer “frame.” The wheel has the brightest stars marked on it and uses the equatorial coordinate system. The frame has been pre-cut to represent the horizon for your latitude on the Earth. By lining up the month and day on the wheel with the time printed on the frame, we are effectively converting between the equatorial and horizon coordinate systems. Remember, to make the conversion, we needed the time and place of the observation – the frame itself takes care of the place and we just dialed in the time. In the center of the cutout area of the frame – not the metal pivot around which everything rotates – is the point directly overhead, called the zenith. One horizon is marked “north”, one is marked “south”, and so on.

The planisphere is just a guide for you to find things in the sky. We've taken a spherical globe and smashed it onto a flat disk, so things will look a little distorted. Because of the distortion, constellations in the sky will not appear as they do on the planisphere, but the planisphere can help us identify bright stars and give us a general idea of where to look for other stars. It's also very useful in figuring out when certain star will rise or set. The best way to get comfortable with the planisphere is simply to take it outside and use it. Like most things, your ability with it will improve with practice.

Using the Planisphere:

1. It is March 7th at 8:00 p.m. and I am looking at Canis Minor. What is the brightest star in this constellation?

2. Which direction am I facing?

3. Which way should I look to find Orion? (up/down/left/right, etc.)

4. Which way should I look to find Leo?

5. Which constellation is closest to my zenith?

6. Is Pisces rising or setting?

7. List one circumpolar constellation (other than Ursa Minor).

8. At what time on April 7th will the Regulus rise?

9. On what date will Sirius set at 9 p.m.?

10. A globular cluster is a tight group of thousands of stars orbiting outside of the disk of the Milky Way Galaxy. What are the right ascension and declination of the M13 globular cluster in Hercules?

1. What time will M13 be at zenith on February 6th?

12. What time will M13 set on February 6th?