Using the Star and Planet Locator

Objective: Using the Star and Planet Locator Laboratory 2 This laboratory introduces the Star and Planet Locator; 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 stars rise and set, as well as when the best time to observe 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 Majoris is the brightest star in the constellation Canis Majoris). 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 2nd 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 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 location in the sky as well as the best time to observe it. The Star Wheel Shown in the figure is the basic outline of the star wheel. This wheel is a two dimensional representation of our sky. To set the wheel rotate the wheel until the date of the observation matches with the desired time. Figure 1 shows the proper rotation of the star wheel to set the sky for 10PM PST on March 14. Note that the times on the star wheel always assume standard time. If the time you are looking is written in daylight savings time, subtract one hour to set the wheel properly. Figure 1 Setting the star wheel to view the sky for 10PM PST on March 14. The sky is represented by the oval at the center of the diagram. Our actual sky, a three dimensional hemisphere is warped into a two dimensional oval. The brass circle about which the stars “turn” is the North Celestial Pole. Notice that as the wheel turns this point always remains at the same position in our sky. The zenith, the point directly over our head in the sky, is at the direct center of the oval. The horizon, the imaginary boundary between the earth and the sky, is represented as the edge of the oval. The stars and constellations which are within the oval would be above our horizon, while any stars which are outside the oval would be beneath our horizon. Figure 2 shows the oval with these positions shown. Figure 2 Representation of the star wheel’s view of the sky. The Z represents the zenith, the O represents the north celestial pole and the red boundary is the horizon. Turning the wheel counterclockwise represents the daily motion of the stars in our sky. Notice the stars rise above the horizon on the eastern side while stars set beneath the horizon on the western side. A line directly from the north to the south through the zenith is called the celestial meridian. This line divides the sky into eastern and western halves. Any star crossing this line is said to be culminating, at its highest point. Figure 3 shows the oval representing the sky with these features marked. Figure 3 Shows the compass directions along with the indications of where stars would be rising, setting and culminating. The green line down the center of the oval represents the celestial meridian. Two other special lines are marked on the star and planet locator. A solid white line indicates the Celestial Equator. In our sky, this line runs from due east to due west along a slanted path. Since the turning of the star wheel follows the rotation of the earth, as does the celestial equator, the line always appears in the same place. The dashed line is the Ecliptic, the apparent path of the Sun in our sky.