Labs from YSI 94 :
Observing the Outer Planets.

Dr. Rich Kron
CARA Yerkes Summer Institute, August 1994

This is meant to be given to the students.

Part I. The Telescope

The telescope in the Yerkes back building is a 10-inch Cassegrain reflector. This means it has a mirror that has a diameter of 10 inches, which is a measure of the "light gathering power." The term cassegrain refers to the fact that there are two mirrors, a primary and a smaller secondary mirror. Look inside the telescope to see how the light travels through the optical system.

There are two components of the telescope that are different from the portable telescopes from the Milwaukee Astronomical Society. First, we have the capability of using a video camera in place of the eyepiece. Second, the position of the telescope can be set using the dials (properly called setting circles) on the two axes. One of these axes is called hour angle, and the other is called declination. Your instructor will show you which is which.

Part II. Sky Coordinates

Positions of stars, planets, galaxies, etc. are reckoned in terms of coordinates, which are called right ascension and declination (just like the axis of the telescope - the reason for this should become clear). Right ascension is often abbreviated RA or alpha (Greek alpha), and declination is often abbreviated dec or delta (Greek delta). The RA is often given in terms of hours, minutes, and seconds, and the declination is often given in terms of degrees, minutes of arc, and seconds of arc. For example, the position of Vega is approximately: alpha = 18h 37m delta = +38° 45'

To set the telescope on Vega, first move the declination axis so that the indicator on the setting circle reads the declination given above (since 45 minutes is 3/4 of 60 minutes, in practice you would set the indicator to 3/4 of the way from "38" to "39"). Setting the right ascension involves an additional step, because of the rotation of the Earth, as follows.

There are two digital clocks; the top one reads local civil time, and the bottom one reads local sidereal time (LST). The local sidereal time tells you the right ascension of a star that is due south (or due north) at that instant. If a star has a right ascension that is, say, 1 hour larger than the sidereal time, that means it will be due south in one hour. We say it is currently "one hour east." What we need on the setting circle is not the right ascension, but rather the difference between the right ascension and the sidereal time, a quantity called the hour angle (HA). The formula is:


The convention is that if the HA is negative, the star is on the east side of the sky, and if it is positive, it is on the west side of the sky. When you set the hour angle on the setting circle, you need to pay attention to which way is east and west.

To summarize: the right ascension of a star is what you find in a catalogue; it does not change (much). The hour angle, on the other hand, is continuously changing because the Earth is rotating, and you need to know it in order to set the telescope. To get the hour angle, you use the formula given above.

Part III. The Planets

In this lab, we will use the setting circles to locate the outer planets:

Jupiter: = 14h 20.2m; = -12° 54'; diameter = 36.0"
Uranus: = 19h 41.4m ; = -21° 56' ; diameter = 3.6"
Saturn: = 22h 51.3m ; = -09° 23' ; diameter = 18.7"

We also need to check the sidereal time. At 9:00 pm, the sidereal time should read:

Tuesday evening, August 9 17h 17.8m
Wednesday evening, August 10 17h 21.7m
Thursday evening, August 11 17h 25.7m


  1. The planets of course move in the sky, but over the three nights the motion will not be very much. The positions listed above are strictly for Wednesday evening, but they should be accurate enough for all three evenings.
  2. The sidereal clock only reads hours and minutes, with no seconds. Therefore, it is unnecessary to have the right ascension accurate to the nearest second. The values listed above give times and right ascensions to the nearest decimal minute, or 6 seconds.
  3. All the planets happen to be south (negative sign in front of the declination). You have to pay attention to this, too, when using the setting circles on the telescope.
  4. Notice that the sidereal time changes by about 4 minutes each day, at a fixed local civil time. Why?
  5. The table includes also the angular diameters of the planets, in seconds of arc ("). These values will be used later.

Part IV. The Project

The point of the project is to demonstrate how you can locate something that is too faint to see with the naked eye, such as Uranus. For things as bright as Jupiter, you can simply sight along the telescope and find it in the small finder telescope, but this obviously won't work for faint stars and planets. The procedure outlined above is exactly what professional astronomers use.

For Jupiter and Saturn, we can make a video tape (Uranus will be too faint, but we can try!). You can use the tape to record the following things for the two planets:

Examine these things and be prepared to answer questions about them. For example, how much larger do you measure Jupiter to be than Saturn (not including the rings - just the disk of the planet)? Does this ratio agree with the values for the angular diameters given in the table above?

You will also notice that the brightness of the surface of Saturn is fainter than the brightness of the surface of Jupiter. Why?

Important Disclaimers and Caveats: