Application Exercise: Using a Planisphere *

Application Exercise: Using a Planisphere*

Objectives

o Become familiar with the night sky through the use of a planisphere or star finder o Be able to define the basic words describing the celestial sphere o Recognize how stars move over the course of a night o List constellations that will be visible at a given date and time o Describe the motion of circumpolar stars and those near the southern horizon o Use the planisphere to estimate the number of daylight hours during the solstices o Identify actual constellations and stars during an observing session o Estimate the angular distances between two celestial objects noted during an observing session

Equipment:

 Any suitable planisphere. (If in doubt, ask the instructor.)  Flashlight (preferably red)

Introduction

There are many ways to locate objects in the sky: seasonal star maps. computer planetarium programs, planetaria (those rooms with stars projected onto a dome), and a planisphere. Literally, planisphere means the sphere (of the sky) projected onto a plane. While star maps may be good for only a month, planetarium programs need a computer, and going to see a planetarium program requires a ticket, a planisphere can be used for any day and time and is easily portable. Plus, planispheres usually cost less than $10.

Vocabulary You Should Know: north celestial pole (NCP), celestial sphere, celestial equator, horizon, zenith, meridian, ecliptic, constellation, star, circumpolar, precession

Using a Planisphere

What Information is on a Planisphere? A planisphere is meant to be a simple representation of the night sky so the planisphere itself must be kept simple. Most planispheres are somewhat small, which limits what can be placed on them. Take a look at the picture of the planisphere that we will be using in this class shown on the following page. On the “holder card” in each corner you see a cardinal point. Around the outside of the inner wheel, you’ll seen the months and days. There is a circle of time, along with arrows, running around the holder card. To see what the sky would look like at any given date and time, simply line up the time with the month and day. The planisphere doesn’t care if it is day or night – it shows what stars are “up” even though we wouldn’t see them if the Sun were out.

The oval hole in the holder card represents the sky. The edge of the oval represents the horizon. Stars outside of this oval represent those you cannot see – they are below your horizon on the date and time you have set. One cannot use a single planisphere for every location on Earth. Generally, planispheres are made for the extreme north or south, mid- latitudes, and equatorial regions. The one we use applies to latitudes between 40 and about 55 degrees north latitudes.

Inside the oval cutout, you will find the stars. For simplicity, the planisphere shows just the brightest stars, those making up the shape of the constellation. These stars are connected by lines showing the basic shape of the

* The overall content for this exercise was based on information found on a now-lost, website. Apologies to whomever put together the original document; it led to this document. constellation. The names of the constellations are given in all capital letters. The names of stars and other objects are given in lower-case letters. The rivet punches through the location of the north celestial pole (NCP), that location on the celestial sphere where the spin axis of the Earth “meets the sky.” The stars appear to rotate about this point (turn the wheel counterclockwise) from East to West as the Earth rotates from West to East.

Running through the stars and constellations are 3 great circles: the celestial equator (solid line), the ecliptic (dashed line), and the Milky Way (fuzzy path).

Compass directions, meridian, zenith: Compass directions, North South, East, and West, are placed on planisphere to help you orient the planisphere correctly. This assumes that you know which direction North is. If you are unfamiliar with the sky you can easily find north by looking for the Big Dipper. The planisphere also shows other constellations that lie near the NCP. Because the planisphere represents a map of the sky, you need to hold it overhead for the compass directions to be correctly oriented.

Two important items not shown on planispheres are the meridian and zenith. The meridian is an imaginary line in the sky that runs from north to south. It divides the sky into east and west halves. A star or other celestial object is at its highest point in the sky (which may still be fairly low in your sky) when it is on the meridian. To find the location of the meridian on your planisphere, picture an imaginary line running from north to south. You can install a permanent meridian on your planisphere by putting a piece of thread between the two “string holes,” being careful not to run the string through the dark blue star wheel itself. Your zenith is an imaginary point in the sky that is straight up from wherever you are. Your zenith is located on the meridian at a declination equal to your geographic latitude.

Setting and Holding a Planisphere: In order to see what the sky will look like on a particular night you must set the planisphere for that date and time. The time scale on planispheres is standard time. During daylight savings time you mush subtract one hour from your time to make it correspond to standard time shown on the wheel (although 1 hour makes only little difference in the stars you will see, affecting those on the eastern or western horizon the most).

When you are using the planisphere to locate objects in the night sky you should hold the planisphere in the proper orientation, so the planisphere matches the orientation of the stars in the sky. The compass directions are labeled on the planisphere to aid in this. If you are looking north, the word NORTH must be pointing down, towards the bottom. The planisphere in the diagram at the right is oriented as it should be if you were looking north. You would then hold the planisphere up above your head to match the stars on the planisphere with those in the night sky. If you wanted to look at stars above the western horizon, you would first face west, and then you must rotate the entire planisphere counterclockwise so that the word “WEST” is on the bottom. As you continue to rotate yourself counterclockwise so that you are facing south, rotate the planisphere counterclockwise as well, and hold it above your head to locate stars and constellations.

Rising, Setting, and Circumpolar Stars: If you rotate the sky in your planisphere, you will see stars “rising up” from along the eastern horizon and setting along the western horizon. Notice as you turn the wheel, that some stars may rise in the northeast or the southeast and set in the northwest or southwest Not all stars rise precisely east nor do all stars set precisely west! The planisphere is helpful in two ways with respect to rising and setting of stars or other celestial objects: We can find the direction in which they rise and set and we can find the rising and setting times.

Let’s pick summer-time objects to use as examples. Turn your planisphere so that it is 10 pm PDT (9 pm PST) on July 31. On the eastern horizon, we see that the Great Square of Pegasus is rising. Towards the northeast, we see a small word “Galaxy” representing the Andromeda galaxy that is just above the horizon. Towards the southeast we see the constellation of Capricornus already risen. Set your planisphere back in time, to 9 pm PST May 15. In the southeast sky, you see the star Antares rising. Turn the inner wheel until Antares is just on the southwestern horizon. At approximately what time does Antares set? Answer: between 5 and 6 am. Remember, we are still talking about May, so we look at May 16 (behind part of the planisphere) and note the approximate time.

Antares is a star that rises and sets every day from our latitude; however, some stars never rise and never set. These stars rotate around the Celestial Pole without “touching” or passing beneath the horizon. These stars are called circumpolar. The celestial sphere has a coordinate system similar to latitude and longitude on Earth, only the “grid” has different names. Declination is analogous to latitude, but runs from +90 (NCP) to -90 (SCP) degrees. Right ascension is similar to longitude; we’ll discuss right ascension more later. To find the declination above which stars are circumpolar subtract your latitude from 90°. In Seattle, at a latitude of about 48° N, all stars above declination 90° - 48° or a declination of +42° will neither set nor rise. To see this, find the constellation Cassiopeia on your planisphere. Now rotate the planisphere while watching the stars in Cassiopeia. You will see that stars that make up the “big W” shape of the constellation never dip below the horizon. This occurs simply as a result of our latitude on Earth. The higher the latitude the more circumpolar stars. If you were to live at the North Pole, all of the stars would be circumpolar. If you are on the equator there are no circumpolar stars, all stars there rise and set (although Polaris, the north star, sits right on the northern horizon). On the other hand, some stars never are seen at all from our latitude. The lowest declination for objects that can be seen from Seattle, at latitude 48° N, is –42° (48° - 90°). Any star with a declination lower than this will never rise above the southern horizon.

Diurnal and Seasonal Motion: If you rotate the star dial through one full day (representing 24 hours), notice that when you move it to the next day, you had to rotate it a little more than one full turn. This is because the Earth has moved almost 1° around the Sun. The time it takes to make one full rotation with respect to the stars is a sidereal day, which is slightly shorter than the solar day of 24 hours. Notice that the stars rise about 4 minutes earlier each night. Diurnal motion (daily motion) can be seen on a planisphere by rotating the star dial through 24 hours. Seasonal motion can be seen on a planisphere by setting it for the same time on successive nights and for the same time on different months. Notice how the stars rise about 4 minutes earlier each night. Procedure

Part 1: Astro Lingo

Using your own words, write down the definitions for the following words or phrases: circumpolar: meridian: zenith: ecliptic: celestial equator: horizon: north celestial pole: constellation (modern definition): Part 2 Becoming Familiar with Using the Planisphere ( 1 point each, unless otherwise noted ) Read through the pamphlet or other information that comes with the planisphere. Become familiar with how the inner circle moves, locate the dates and times, horizon, compass directions, etc.

1. What is the current date and clock time? ______Convert this to Standard Time: ______Example: 10 pm Pacific Daylight Time (PDT) is 9 pm Pacific Standard Time (PST)

Now, set your planisphere to show the sky at 9:00 pm PST (21:00) on January 26th.

2. On a clear night which of the following constellations would be entirely visible above the horizon on this date and time? o Ursa Major o Canis Major o Aquarius o Aries o Andromeda

3. Which direction would you look to see the star Procyon (N, NE, E, SE, etc.)? ______

4. Which named star will be most directly overhead (at your zenith)? ______

Follow the constellations through one complete rotation of the dial, counterclockwise. Notice that some constellations never set. These are called circumpolar constellations.

5. List three circumpolar constellations. a. ______, b. ______, c. ______

6. How would the number of constellations that are circumpolar vary with your latitude? (Outside research recommended.) ______

______

7. Describe the path that stars move close to the north celestial pole. ______

8. Describe the path that stars move near the southern horizon, moving from SE to SW. ______

9. (2 pts) How many degrees does a star in the sky rotate about the celestial pole each hour? ______, in a day? ______How is your answer related to how many degrees the Earth rotates each day?

______

______Locate the ecliptic on the planisphere (dashed line). The Sun moves only slightly on the ecliptic in any one day, taking a full year to circle it once, from west to east. Imagine the Sun at the summer solstice on June 21 – it would be in between the constellation of Taurus and Gemini. Look for the time lined up with June 21 when that location in the sky is on the eastern horizon to find the time of sunrise. Similarly find the time of sunset (that location in the sky is on the western horizon).

10. What time does the Sun rise when it is farthest north of the celestial equator? ______

11. How many hours of daylight would you expect on this date? ______

Repeat the above for the winter solstice, December 21, when the Sun is in between Sagittarius and Scorpio:

12. What time does the Sun rise when it is farthest south of the celestial equator? ______

13. How many hours of daylight would you expect on this date? ______

Part 3: Observing the REAL Night Sky (1pt each unless otherwise noted) Set your planisphere for the time and date you are actually observing. Remember to convert daylight time to standard time if necessary. When looking for stars and constellations in the sky, be sure to determine your direction: north, east, south, or west. Turn the planisphere so the direction you wish to observe is at the bottom, pointing down. Follow the instructions below.

1. List the date and time of this activity. Use am, pm or 24-hour format, and give the full time zone label

(e.g., PST, MST, CST, EST)

2. Face north. List four constellations that you can see in the northern sky. a. ______b. ______c. ______d. ______

3. Now find Polaris, the North Star. It is the end star in Ursa Minor. On the planisphere it is the rivet. How many stars can you see in Ursa Minor1?

4. (2 pts) Find 5 bright stars in the sky. List their names and the constellations they are in. If the star is not named on the planisphere, sketch the constellation and the star’s location to look up later (see note at the very end of this exercise).

Star name: a. b. c. d. e.

Constellation: a. b. c. d. e.

Which of the five stars listed above appears brightest? ______Faintest? ______

1 (Three stars are bright enough to be easily seen; the remaining stars require a clear dark sky, usually away from city lights. The stars in this constellation are good measures of the “seeing.”) 5. Pick two of those stars and measure their angular separation using hand-angle measurements (see next page).

Star 1. ______Star 2. ______Separation:

CONSTELLATION SKETCHES:

Hold your hand out at arms length, level with your eyes. These angular measures are all approximate. Thus, there is no need for any answers such as: 11.67 degrees!

The following web site has a complete list of constellations, with links to star names or identifications in each constellation. You will also find links to other web sites where additional star maps can be found.

http://www.astro.wisc.edu/~dolan/constellations/constellation_list.html