An introduction to Astronomy

Some Examples: Convert from UT to PST: 1.) Convert from 21:35 UT on May 21 to PST 21:35 UT - 8:00 = 13:35 PST on May 21 2.) Convert from 6:35 UT on May 21 to PST 6:35 UT - 8:00 = -2:35 = 22.35 PST on May 20. If the result is negative subtract the hours from 24. Convert from PST to UT: 3.) Convert from 14:35 PST on May 21 to UT 14:35 PST + 8:00 = 22:35 UT on May 21 4.) Convert from 18:35 PST on May 21 to UT 18:35 PST + 8:00 = 2:35 UT on May 22 Time Unit Conversion: Converting from hours, minutes and seconds into just hours expressed in decimal form, and vice versa: 5h38m = (5 + 38/60)h = (5 + 0.633)h = 5.63h 5.63h = 5h and (0.63 x 60)m = 5h37.8m 5h37.8m = 5h and 37 min and (0.8 x 60) = 5 h 37m 48s Arc Unit Conversion: Converting from degrees and arc minutes into just degrees expressed in decimal form, and vice versa: 24°26' = (24 + 26/60)° = (24 + 0.43)° = 24.43° 24.43° = 24° and (0.43 x 60)' = 24°25.99' = 24°26' 2 Language Objective: This activity introduces coordinate systems that are used in astronomy to describe the positions of celestial objects in space. Background: Consider a situation in which you are observing a celestial object and wish to communicate to another person where to look in order to view the same object. How would you describe it if you were speaking to somebody over the phone? Would you need a different description if they were located in New York instead of Los Angeles? To fix the position of astronomical objects in space, we need a frame of reference, or coordinate system, which assigns a unique set of numbers to each point in the sky. Each coordinate system we will introduce has a particular use in Astronomy. The first coordinate system we will introduce is the Terrestrial System. This is the familiar system of Latitude and Longitude that locates positions on the Earth. The observer’s location on Earth determines the sky they will see. The sky appears quite different at say the North Pole and the Equator. The second system, the Horizon System, describes the position of objects local to an observer at a given position on Earth. We only need Azimuth and Altitude to fix a star’s location in the sky at a given time. The third system, the Equatorial System, fixes the position of astronomical objects on the celestial sphere. The Terrestrial System This coordinate system is based on Latitudes and Longitudes, which are commonly used for navigation on Earth. The terrestrial coordinate system is shown in Figure 1. Latitude: Lines of latitude are measured ° N or ° S (degrees North or degrees South). Lines of latitude run east and west. The Earth’s equator is 0° latitude. The North Pole is 90° N latitude and the South Pole is 90° S Latitude. All lines of latitude are parallel to each other. Longitude: Lines of longitude are measured in ° E or ° W (degrees East or degrees West). Lines of longitude run north and south. The Prime Meridian, or the line of 0 degrees longitude, runs through Greenwich, England. Longitude is usually specified as an angular measurement East or West of Greenwich. Lines of longitude meet at the north and south poles. There are 24 lines of longitude. It takes approximately 24 hours for the Earth to rotate once on its axis. Recall that there are 360° in a circle. Since 24 divides into 360° 15 times, each line of longitude is 15° apart. 3 Language SKY MOTIONS As we look toward the sky, we notice the positions of the sun, moon, stars and planets all change over time. We understand that these apparent motions are due to the motions of the Earth. The earth's rotation on its axis is responsible for the daily motion of all celestial bodies across the sky every 24 hours. The earth's revolution around the sun is responsible for the sun's apparent motion through the constellations and having different constellations appearing at evening hours during different times of the year. However, for the general population, these conclusions are drawn from looking at the sky once and then once again several hours later and seeing that the sun, moon or stars have changed position.

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