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The radius of an ellipse such that it is half the longest diameter.

4.

The plane of the earth's orbit around the sun.

6.

An astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes.

7.

In geometry, a pair of special points with reference to which any of a variety of curves is constructed.

9.

A circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic.

10.

Occuring twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator.

Down:

1.

Approximately 186,282 miles per second.

2.

A unit of length approximately equal to the mean Earth–Sun distance.

3.

The angular separation between two objects as perceived by an observer.

5.

The radius of an ellipse such that it is half the shortest diameter.

8.

A displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.