Using the Star Wheel Laboratory 2

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1 Objective: Using the Star Wheel Laboratory 2 This laboratory introduces the Star Wheel; which is a common tool used in backyard observing. This tool helps approximate the location of constellations and certain stars as they move across the sky during a single night (this is called daily motion). In addition, it is possible to approximate the time that a star will rise and set, as well as when the best time to observe it will be. Background: Constellations Gazing at the night sky on a dark clear night we can see several thousand stars scattered in random patterns or groups. These groups have been noticed and are called constellations. To ancient sky gazers a constellation was a loose group of stars that represented a certain figure or pattern. They gave these star patterns names after their heroes (such as Orion, Hercules, Cassiopeia ); other constellation names cam from animals (Taurus, Leo ). Almost half of all today s known constellations were invented in ancient times. More recently 40 new constellations have been added by astronomers to make a new total of 88. Modern astronomers have assigned each constellation definite boundaries; thus today a constellation represents not just a group of stars but also an area of the sky. Any star within the specified area is apart of one particular constellation. However it should be kept in mind that the stars in a single constellation are not physically associated with each other. Some stars of a single constellation are many times farther away than other, and furthermore all the stars may be moving in different directions. The only thing that all such stars have in common is that they all lie approximately in the same direction when observed from Earth. Finally, today s constellations have only one purpose to indicate directions or locations in the sky. Stars Ancient sky gazers also named some of the brightest stars in the sky, and modern astronomers still use many of these names. Several such names are Sirius, Vega, Deneb, Altair, Spica, Betelgeuse, Rigel, Arcturus, Aldebaran, Antares, and so on. However, there are not enough names for the thousands of stars we can see, and the listed names do not help us locate the stars in the sky. Modern astronomers have named stars in such a way that Greek letters are assigned to represent the brightness. Thus the brightest star in a constellation is designated α (alpha), the second brightest β (beta), the third brightest γ (gamma), the fourth is δ (delta), and so on. This identifies the star and constellation and gives us a clue to the brightness of the star (α Canis Major is the brightest star in the constellation Canis Major). Compare this with its ancient name, Sirius, which tells nothing about the stars location or brightness. Probably the most famous star in our sky is not one of the very brightest stars that we can see but a star of only 2 nd magnitude. It is located at the very end of a small constellation whose proper name is Ursa Minor; better known as the Little Dipper, and 7

2 the star is Polaris, better known as the North Star. We call it the North Star because it is located almost directly above the Earth s North Pole and therefore almost at the North Celestial Pole. If you are able to find it, then you are facing due north. Zenith and Meridian The best time to observe a star or any celestial object is when it is in the observer s zenith (the position directly above the observer). Due to the rotation of the Earth and the position of the celestial objects there are few objects that will pass through the observer s zenith. In the case where an object will never be in the zenith it is best to observe that object when it crosses the observer s meridian, a great circle that passes directly through the zenith. Practical Use Suppose that you want to observe the galaxy Andromeda but you don t know where it is in the night sky or what the best time to observe it will be. What you do know is that it is in the constellation Andromeda. Locating the constellation Andromeda on the Star Wheel will help you identify its position in the sky as well as the best time to observe it. Now you would like to know when a particular star will rise, cross the meridian, and set on a given date. To find the time that a star will rise physically rotate the star wheel so that the star is as far east as possible. Locate the date in question and read off the corresponding time. Do the same for determining when a star will set, except this time make sure the star is as far west as possible. For determining when a star crosses the meridian, rotate the star wheel so that the star is on the meridian, locate the date in question and read off the corresponding time. 8

3 Name: Date: Using the Star Wheel Lab 2: Data Sheet Consider only the indicated patterns of stars as individual constellations. The meridian must cross through the indicated pattern to be passing through a constellation. Also, more that half of the constellation must be visible or invisible to be considered for an answer. 1) Set the Star Wheel locator to 8 pm on the assigned date: a. Which constellations are on the meridian? b. Is there a constellation in the Zenith? If yes, which one? 2) Turn the Star Wheel locator to 1 am the next morning: a. Which constellations are on the meridian? b. Is there a constellation in the Zenith? If yes, which one? 3) What constellations will be at 8 pm: a. In the meridian 3 months from the assigned date? 9

4 b. In the zenith 3 months from the assigned date? c. In the meridian 6 months from the assigned date? d. In the zenith 6 months from the assigned date? e. In the meridian 9 months from the assigned date? f. In the zenith 9 months from the assigned date? 4) The constellation has a bright star named. With the Star Wheel locator find for the assigned date, and for 3 and 6 months later, the times when the star rises, crosses the meridian, and sets. Assigned date 3 months later 6 months later Date Rises Crosses Meridian Sets 10

5 5) Turn the Star Wheel several times around. Can you find any constellations that do not disappear when you turn the wheel completely around? If yes, such constellations are referred to as circumpolar, list them. 6) Why do stars undergo their daily motion in the sky? 7) Why are there different stars and constellations seen in the sky during each season of a year? 11

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