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Happy Autumnal Equinox!

At 10:21 am Eastern Daylight Time on Thursday, September 22, the Autumnal Equinox occurs, and the northern hemisphere autumn officially begins! Here’s why…

At any given time, from our point of view here on Earth, the Sun is situated in front of the distant background stars. As we move through one orbit of the Sun every year, the Sun appears to shift eastward through those fixed stars. In fact, this is where the Zodiac originated. Early sky watchers noted that, in the course of one year, the Sun travelled through twelve of the constellations, landing in the same one every year on the same date. Nowadays, every year in the third week of September, the Sun is among the stars of the constellation Virgo the Maiden.

The Earth’s annual journey causes the Sun to trace a great circle around the sky called the Ecliptic. The major Solar System bodies are always found near this imaginary track in the sky because it is close to the plane of our Solar System as projected onto the sky.

Now, imagine a second circle positioned directly over the Earth’s Equator, and painted as a strip around the sky, and dividing it into two great bowls, the Northern Hemisphere and the Southern. Since the Earth’s equator is south of observers in the Northern Hemisphere, the equator circle in the sky is always to their south, from the eastern to the western horizon.

Because the Earth’s axis of rotation is tilted 23.5° away from the ecliptic, the equator band and the ecliptic band behave like two hula hoops with the same centre, but tilted so that they intersect at only two spots. At the moment of the Autumnal Equinox, the Sun is situated at one of the intersection points — and its apparent motion is carrying it into the southern bowl of the sky. Six months from now, on the Vernal Equinox, it will again cross the Equator at the other intersection point, heading into the northern bowl of the sky, and northern Spring will begin.

The equinox triggers a few interesting effects. First of all, for the next six months, the Sun will spend all of each day in the southern hemisphere sky, overhead of the lucky folks who live there! With the Sun higher in their sky, they experience more daylight hours and receive more concentrated solar radiation, producing warmer air. (Compare the intensity of a flashlight beamed straight at a wall versus obliquely at the wall. The bright circle of the beam gets weaker as it spreads into an oval.) At the same time, North Americans, Europeans, and Asians have to accept shorter, colder days and longer nights (which are great for warmly dressed astronomers). Secondly, on the day of the equinox, everyone worldwide experiences 12 hours each of daytime and night-time. This is where the word equinox (Latin for equal night) comes from.

The times around the equinoxes offer better chances to see the aurorae at high north and south latitudes, too. Just as two bar magnets lined up with their poles in the same direction repel one another strongly, the Earth’s magnetic field repels the Sun’s field. At the equinoxes, the Earth’s axis tilts neither towards nor away from the Sun, so the two “magnets” aren’t lined up as well, reducing Earth’s ability to deflect the Sun’s field and the charged particles that trigger aurorae in our upper atmosphere.

The article by Chris Vaughan.

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