![]() ![]() Each day the Sun's daily arc moves northward or southward, depending on the time of the year. From our perspective, the Sun follows the ecliptic in its path through the sky throughout the year. Earth's rotational axis does not point straight up and down, like the handle of a perfectly spinning top, but is slanted about 23½° with respect to our orbit around the Sun.Īnother way to think of this is that the plane defined by Earth's orbit around the Sun (called the ecliptic) is tilted with respect to the planet's equator. The apparent position of the Sun in our sky is farther north or farther south depending on the time of year due to the globe's axial tilt. Equinoxes occur when the Sun crosses from northern declinations to southern ones, or vice versa. One consequence: the celestial-coordinate system is tilted 23½° with respect to the ecliptic (the path followed by the Sun through the stars over the course of a year). Earth's spin axis isn't perpendicular to the plane of its orbit around the Sun. This seems awfully precise for seasons that gradually flow from one to the next, but the reason astronomers regard this event as the “End of Summer” and “Beginning of Fall” is because it is marked by a key moment in Earth’s annual orbit. For us northerners, this event is called the autumnal equinox. At that moment, the Sun shines directly on Earth’s equator, heading south as seen in the sky. Astronomically speaking, however, the fall season only comes to the Northern Hemisphere on Tuesday, September 23rd at 2:29 Universal Time (10:29 p.m. ![]() To those who’ve unpacked their winter coats, closed their windows at night, and felt that telltale crispness in the air, it seems that autumn has already started. What is the "fall equinox" - and how do we know when it happens? ![]()
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