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Astronomers can use an equatorial coordinate system to map' our sky. Imagine a giant ball or sphere surrounding our planet. This sphere is called the celestial sphere celestial means things that have to do with the sky or the heavens. Line up this sphere with the north and south poles of earth and you will find the sphere's celestial poles. Now draw a line that shows the Sun's movement in accordance with the stars for the entire year. This line or path is called the ecliptic. At a 23.5tilt, the ecliptic does not line up with the celestial equator, in fact, it crosses the celestial equator at 2 points on opposite sides of the celestial sphere. The places where these lines cross are called the vernal equinox, marking the beginning of spring, and the autumnal equinox, marking the beginning of fall. The summer solstice, marking the beginning of summer, and the winter solstice, marking the beginning of winter, are the points at which the ecliptic and the celestial equator are farthest from one another. So we know that on June 21 in the northern hemisphere of the earth, we are seeing where the ecliptic and the celestial equator are at their farthest apart. This is the longest day of the year, and days will begin to get shorter afterward as we move toward the place where the lines cross at the autumnal equinox.

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