Labs from YSI 94 :
Dr. Rich Kron
Observing the Outer Planets.
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
like the axis of the telescope - the reason for this should become
clear). Right ascension is often abbreviated RA or
and declination is often abbreviated dec or
(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:
= 18h 37m
= +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
between the right ascension and the sidereal time, a quantity called
the hour angle (HA). The formula is:
HA = LST - RA
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
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
- 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.
- 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.
- 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.
- Notice that the sidereal time changes by about 4 minutes each
day, at a fixed local civil time. Why?
- 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?
- diameter of the image
- motion of the image
- features on the surface of each planet
- Galilean satellites of Jupiter
- rings of Saturn.
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