DSST Astronomy

# Xplanation celestial the celestial sphere can be

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xplanation: Celestial. The celestial sphere can be thought to be a hollow shell of extremely large radius centered on the observer. Coordinate systems, analogous to latitude and longitude can be devised to designate the positions of objects in the sky. By imagining that the meridian of the observer on the Earth is projected upward and outward until it reaches the celestial sphere, it then becomes the observer's celestial _________________. Explanation: Meridian. As you can see in the figure below, the reference points we use on the earth are projected outward onto the celestial sphere, so the Earth's equator is projected outward to form the celestial equator, the north and south poles project outward to form the north and south celestial poles, and the meridians on earth are projected outward to form celestial meridians:

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The celestial sphere stays still; it's the Earth that rotates. As the Earth turns, the observer's terrestrial meridian moves under the celestial sphere and the stars pass by the observer's stationary celestial meridian. The _____________ is the overhead point along the extension of a line from the center of the Earth up through the observer. ailed Explanation: Zenith. The zenith is the point on the celestial sphere that's directly over our heads at any given time. Together with the horizon, the zenith is used to denote the position of objects in the sky. The ____________ is a great circle on the celestial sphere 90° from the zenith. Explanation: Horizon. The horizon passes through the center of the Earth and is parallel to the sensible horizon of a given position or the plane of such a circle. Keep in mind that the zenith and horizon are relative to where on Earth a person is standing; the zenith for a person in Seattle, Washington will not be the same as the zenith for someone in Seoul, Korea:
A system that is relative to the observer for specifying the position of a star is the ______________ coordinate system. Explanation: Horizontal. The horizontal coordinate system uses measurements relative to the observer's horizon, therefore two people standing in two different parts of the world will have different measurements for the same star. The system consists of two measurements--the azimuth and the altitude. The azimuth runs from 0° to 360° and tells the observer what direction to look in, while the altitude tells the observer how high to look. The plane in which the Earth revolves is 23.5° different than the plane of the equator and this is called the ____________ of the ecliptic. Explanation: Obliquity. The ecliptic plane is the imaginary plane defined by the projection of Earth's orbit (the path the Earth follows around the sun) onto the celestial sphere. As you know, the Earth is tilted; as shown in the graphic below, the difference between the plane that the equator is in and the ecliptic plane is 23.5 degrees: While the Earth’s orbit around the sun varies by about 3% (about 91.5 million miles from the Sun at the closest point and 94.5 million miles at the furthest point of the orbit), it is the obliquity of the ecliptic that causes the variance in the seasons on Earth.

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