Download Lab 2: The Planisphere

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Lab 2: The 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: Use your planisphere to answer the following questions. When
a question asks you to give a time of day, estimate the time to the nearest quarter hour
(*:00, *:15, *:30, or *:45) and specify if it is AM or PM. Also, for this lab, use
noon=12:00PM and midnight=12:00AM, or just write out “noon” or “midnight.” The
questions with *** are to be answered if the weather is bad; if the sky is clear, don't
bother answering those, as you will instead take that time to go outside and look at the
constellations.
1. It is September 10th at 8:00 p.m. and I am looking at Aquila. What is the brightest star
in this constellation?
2. Which direction am I facing?
3. Which way should I look to find Cygnus? (up/down/left/right, etc.)
4. Which way should I look from Cygnus to find Hercules?
5. Which very bright star is closest to my zenith? What constellation is it in?
6. ***Cygnus, Aquila, and Lyra each contain one very bright star. During the summer
months, these three stars form a triangle. What are the names of these stars? What is
the name of the triangle they form?
7. ***Another name for Cygnus is the “Northern Cross.” This cross marks the location
and direction of the Milky Way across the sky. Looking north from Cygnus, there are
other constellations that appear to lie in, or partially in, the plane of the Milky Way.
Name two of these constellations.
8. Is Pisces rising or setting?
9. List one circumpolar constellation (other than Ursa Minor).
10. At what time on October 15th will the star Betelgeuse rise?
11. On what date will Sirius set at 9 p.m.?
12.
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 (in hours and minutes)
and declination (in degrees) of the M13 globular cluster in Hercules?
13. What time will M13 rise on November 5th?
14.
15.
What time will M13 set on November 5th?
***M31, or the Andromeda Galaxy, is the nearest large galaxy to the Milky Way.
Find it on your planisphere. What constellation is it closest to?
16.
*** On a clear, dark night, it is possible to see M31 with the naked eye.
However, even on such a night, it will appear very faint, so you will need to use
nearby stars in order to find its location. Imagine it is midnight on September 20th.
Starting from the Great Square of Pegasus and using a technique called “star-hopping”
(moving from one star to the next toward your celestial object of choice), describe
how to find the Andromeda Galaxy.