Download Celestial Sphere Lab

Astronomy
The Celestial Sphere Lab
Name_________________________
Date_________
Block________
(This lab has been modified from a University of Michigan Astronomy Department lab.)
Introduction
The ancient Greeks contributed much to the science of astronomy; however, many of the
ideas they proposed have since proven to be incorrect. Some of the concepts they
developed are still useful today though. One of the more useful ideas proposed by the
ancient Greeks is the idea of a celestial sphere. We now know that the Earth’s rotation
causes the stars to appear to move around us in the night sky, but the celestial sphere
concept is still helpful in understanding the motions of the Sun and stars as we see them
from Earth.
Part 1: The Equatorial Coordinate System and the Ecliptic
This section of the lab focuses on the equatorial system of locating objects in the night
sky. Astronomers use Right Ascension (RA) and Declination (Dec) in the sky similarly
to the way we use latitude and longitude here on Earth. Lines of RA are like lines of
longitude and run from the north celestial pole to the south celestial pole. (The north and
south celestial poles (NCP) and (SCP) are just projections of Earth’s north and south
poles out into space.) Lines of Dec are like lines of latitude and run parallel to the
equator. For example, Dec 40 degrees passes directly over the 40 degree latitude line on
Earth.
The Earth rotates once every 24 hours. Since it makes a complete circle in that time, we
can say that 360 degrees = 24 hours. How many degrees are there in one hour?
_________ degrees = 1 hour
Because the number of degrees and the number of hours can be set equal to each other,
astronomers say that RA is in hours instead of degrees. 0 hours RA is defined as the
Vernal Equinox on approximately March 21. The hours of RA increase as eastward so
that objects with larger RA rise later. Dec 0 degrees is the celestial equator and increases
as you move toward the NCP. Dec becomes negative as you move toward the SCP.
The ecliptic is the plane of Earth’s orbit around the sun. It is the path that the Sun travels
in our sky. If the Earth were not tilted, the ecliptic and the equator would match up, but
because the Earth is tilted at 23.5 degrees, the ecliptic is tilted 23.5 degrees from the
equator. In the second section of the lab, we will discover how this tilt of the Earth
causes the seasons to change.
On the celestial sphere, please locate the following and check them off as you find them:
1. NCP
2. SCP
3. Celestial Equator
4. Vernal Equinox
5. Autumnal Equinox
6. Summer Solstice
7. Winter Solstice
8. Ecliptic
Please answer the following questions by observing your celestial sphere:
9. What is the Dec of the NCP?
10. What are the coordinates (RA and Dec) of the summer solstice?
11. What are the coordinates (RA and Dec) of the vernal equinox?
12. Which star is located at the coordinates 5h55m RA and +7degrees24m Dec?
13. Which constellation is near the coordinates 17h RA and -40degrees Dec?
In which constellation is the sun located on the following days?
14. July 5
15. January 20
16. November 10
17. your birthday
Part 2: Location and Motion of the Sun
In this part of the lab, we will explore how the motion of the Sun changes from season to
season. You can simulate the daily motion of the Sun (called the diurnal motion of the
Sun) by rotating the globe. This motion is due to the rotation of Earth. The Earth rotates
west to east but it revolves around the Sun so the stars and the Sun all seem to rise in the
east and set in the west. This makes the Sun appear to move eastward along the ecliptic.
To describe the motion of the sun, we note its position when it rises and when it sets, and
its meridian altitude. The meridian is the line which runs due north-south and passes
through zenith. The altitude of an object is its height in degrees above the horizon.
When the Sun crosses the meridian, it is local noon, and the Sun is at its highest point in
the sky.
In the figure above (not to scale), the person is standing in Ann Arbor, Maine, at a
latitude of about 40 degrees (Mt. Sterling, Kentucky is at a latitude of 38 degrees). The
horizon divides the sky into the half which is visible and the half that is not visible.
There are 90 degrees between the zenith and the horizon. Please fill in the diagram
below with the correct degrees for Mt. Sterling, Kentucky.
Please use your celestial sphere’s calendar and its Sun to fill out the short table below.
Date
March 21
June 21
Sept 21
Dec 21
First Day of …
Declination of the Sun
Questions:
1. Are there any stars that you can never see at your location? If so, describe where
they are using coordinates. Where would you have to go to see some of these
stars?
2. Are there any stars that cannot be seen from the equator? Why?
3. Are there any stars that can never be seen at the North Pole? Why?
Please use your celestial sphere to find a star at the following declination.
Declination of Star
- 30 degrees
0 degrees
+ 30 degrees
Name of Star