Lab 2: the Planisphere

Name: ______

Section: ______

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.” Hopefully the sky will be clear so you can practice what you've learned outside!

1. It is February 6th at 9:00 p.m. and I am looking at Canis Major. 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 from Orion to find Taurus?

1. Which very bright star is closest to my zenith? What constellation is it in?

1. (Planisphere doesn't need to be set to Feb. 6 at 9pm for this.) Cygnus, Aquila, and Lyra each contain one very bright star. During the summer months, these three stars form the “Summer Triangle.” What are the names of these stars?

1. In the summer, you can use Cygnus to find the Milky Way. But Cygnus is below the horizon most of the night in the winter. Set your planisphere back to Feb. 6 at 9pm. Cygnus is setting. From Cygnus in the NW, follow the Milky Way toward the SE horizon. Name 2 constellations that are visible tonight which lie along the Milky Way. Knowing this will help you find the Milky Way in the winter when the sky isn't dark enough for it to be easily seen.

8. Is Pegasus rising or setting?

9. List one circumpolar constellation (other than Ursa Minor/Little Dipper).

10. At what time on October 15th will the star Betelgeuse rise?

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

1. 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?

1. An open cluster is a region where stars are forming, but are not necessarily gravitationally bound to each other. A classic example is the Pleiades, located just north of Taurus. What time will the Pleiades rise on March 10?

1. What time will the Pleiades set on March 10?

1. 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?

1. 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 8pm on Feb. 6th. 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. Assume you are facing north, so Pegasus is to the left of the North Star.