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Unformatted text preview: Astronomy 101 - Test 1 Review FOUNDATIONS Scientists use the metric system to measure things. It is based on powers ten, and is thus more logical than our everyday Imperial system. The kilogram (or gram), meter (or centimeter), and degree Celsius are the basic units. The Kelvin scale is also widely used for temperature. Density is how tightly packed the matter in an object is: higher density means more mass in a given volume. Angles are usually measured in degrees, arcminutes and arcseconds. We use scientific notation a lot in astronomy because very large and small numbers occur frequently. The range of distances, masses, ages and times, and temperatures encountered in astron- omy are enormous. The Astronomical Unit, the light year, the Earth Mass and the Solar Mass are some of the special units used in astronomy in addition to the metric system. The Celestial Sphere is an ancient but still useful concept that allows us to navigate around the night sky, analogous to how we navigate the Earth with longitude and latitude. It is fixed to the stars so it does not rotate with Earth, but appears to rotate once a day as the Earth spins on its axis. It has an equator and poles. Polaris, or the Pole Star is a star located above (but a very long distance from!) Earths North Pole, so that as the Earth rotates, the Celestial Sphere appears to rotate around this star. The Solar Day is how long it takes for the Sun to return to a given position in the sky. The Sidereal Day is how long it takes the Earth to spin 360 degrees on its axis. These are not the same, because as the Earth spins, it also revolves around the Sun. Thus it must rotate a bit more than 360 degrees in order for the Sun to appear in the same position in the sky again. The Solar Day is therefore slightly (4 min) longer. Astronomers need to keep track of sidereal time because where an astronomical object appears in the night sky depends on where Earth is in its daily rotation. This allows astronomers to track, for instance, whether the object is moving in the sky. Seasons exist because the Earths rotation axis is tilted with respect to its orbit around the Sun. The summer solstice in the northern hemisphere occurs when the Earths North Pole is at its most tilted towards the Sun, and so on. The year we use is slightly shorter than the time it takes the Earth to complete exactly one orbit around the Sun. The difference is a matter of minutes. This is because the Earths tilted rotation axis slowly precesses, or wobbles like a spinning top. It takes 26,000 years to do a complete wobble; in other words, while Polaris is the Pole Star now, over time it will slowly circle away from the North Celestial Pole, and return to it in 26,000 years. This means that if we used the Earths orbital period around the Sun as our definition of a year, then in 13,000 years, July would be a winter month. We instead choose to define the year so that the summer solstice occurs at the same time every year, but this occurs at a slightly...
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This document was uploaded on 12/13/2011.
- Fall '09