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| The rate at which an object's velocity changes. Its standard units are m/s^2 |
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| A measure of the angle formed by extending imaginary lines outward from our eyes to span an object (or between two objects) |
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| The point at which an object orbiting the Sun is farthest from the Sun |
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| Direction around the horizon from due north, measured clockwise in degrees. For example, the azimuth of due north is 0 degrees; due east is 90 degrees. |
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| continually visible above the horizon during the entire 360 degrees of daily travel; a circumpolar star always remains above the horizon for a particular latitude |
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| A measure of how much an ellipse deviates from a perfect circle; defined as the center-to-focus distance divided by the length of the semi-major axis |
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| The plane of the Earth's orbit around the Sun |
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| Ptolemaic cosmology includes a smaller circular planetary motion as well as a larger circular orbit around the Earth. Epicycles are used to correct for discrepancies between observed planetary positions and the theoretical positions required for a simple circular orbit. |
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| Anything that can cause a change in momentum |
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| the force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface |
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| A boundary that divides what we can see from what we cannot see |
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| An eclipse that occurs when the Moon passes through the Earth's shadow, which can occur only at full moon; may be total, partial, or penumbral. |
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| The law expressing the force of gravity between two objects, given by the formula F=G[(m1m2/d^2)] |
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| The apparent shifting of an object against the background, due to viewing it from different positions |
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| The geadual wobble of the axis of a rotating object around a vertical line |
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| Motion that is backward compared to the norm. For example, we see Mars in apparent retrograde motion during the periods of time when it moves westward, rather than the more common eastward, relative to the stars |
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| Half the distance across the king axis of an ellipse; usually referred to as the average distance of an orbiting object |
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| A solstice is an astronomical event that happens twice each year, when the tilt of the Earth's axis is most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme |
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| The rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are the same |
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| An event in which a planet passes in front of a star (or the Sun) as seen from Earth. Only Mercury and Venus can be seen in transit of our Sun. The search for transits of extrasolar planets is an important planet detection strategy |
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