On Sunday, Sept. 22, at exactly 4:44 p.m. Eastern Time, the Sun will cross over the celestial equator, signalling the change of seasons — summer to fall in the northern hemisphere and winter to spring in the southern hemisphere — as planet Earth observes the autumnal equinox.
'Celestial equator'? 'Autumnal equinox'? What's this all about, though?
The seasons that we see here on Earth depend two things: the axial tilt of the planet and exactly where the planet is in its orbit around the Sun.
The axis of our planet — the imaginary line that our planet rotates around — isn't straight up and down relative to the solar system. If we trace the path of Earth's orbit around the Sun (creating 'the ecliptic'), and draw a line straight up and down through that path, the line of Earth's axis is tilted away from that straight line by about 23.4 degrees. Astronomers call that angle Earth's axial tilt or obliquity, and it stays the same the whole year as we go around the Sun.
This tilt is the reason why we have seasons. If our axial tilt was closer to zero, like Mercury's is, we wouldn't have seasons, and the life here would probably be a lot different than it is today.
While the tilt causes us to have seasons, which season it is depends on where Earth is in its orbit around the Sun. Since the tilt faces the northern hemisphere more towards the Sun from late March and late September (with the most direct facing in late June), the northern hemisphere has spring and summer and the southern hemisphere has fall and winter. From late September to late March, it's the southern hemisphere that's facing more directly towards the Sun (with the most direct facing in late December), and the seasons are reversed.
However, there are exactly two times when both hemispheres get an equal share of the Sun's light. That's on the days of the equinox.
Right on March 21 or 22 and Sept. 21 or 22 (it varies from year to year), the Sun rises, crosses the sky, and sets along a line that traces Earth's equator almost exactly. In March, this is called the vernal equinox, and in September it's called the autumnal equinox. These days also mark the time when, from our perspective, the Sun crosses the celestial equator — the imaginary line in the sky directly above the Earth's equator — going northward in March and southward in September.
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If you'd like to see a really cool, interactive simulator for all this, click here. This simulator, hosted by the University of Nebraska-Lincoln, lets you move the Earth along its orbit, while at the same time, seeing the angle of sunlight hitting the Earth, both from a perspective in orbit and from on the ground.
Also, you may have heard that you can balance an egg on its end on the equinoxes, but you have to wait for exactly the right time to do it (apparently this is mentioned most for the vernal equinox for some reason). Go ahead and give it a try. However, note that you can do this at any time of the day, on any day of the year. It's not easy to do, but the only trick to balancing an egg on its end is to have patience and persistence. There's no celestial imbalance to overcome. You're just working with Earth's gravity and your own skill. The equinox has nothing to do with it.
(Images courtesy: Matt Cardy/Getty Images, Wikimedia Commons, NASA)
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