My Dark Sky

To me, today – the Autumnal Equinox – is another “milestone day” in a year. This is the time when I sit back and think what have I accomplished or going to accomplish this year, and how much time I’m left with.

Equinox happens once every 6 months – Vernal Equinox in March and Autumnal Equinox in September. Midway between the two equinoxes we have the solstices.

Technically equinox is the time when the Sun crosses the celestial equator. For autumnal equinox, the Sun moves from north to south.

If you have ever paid attention to the direction of the Sun each morning on your way to work, you will observed that the rising Sun is constantly changing its direction everyday, moving sometimes from northeast to the southeast daily, sometimes the other way round. This is especially obvious if there is a building or tree as a reference point: the Sun doesn’t rise the same direction everyday.

The reason is simple: our Earth is tilted. This is exactly the same reason as why we have seasons.

Although we usually said that the Sun rises in the east and set in the west, in reality the Sun only rises exactly due east and set exactly due west only 2 times in a year, that is during the equinoxes. So today, the Sun rises exactly east and will set exactly west.

Refer to the diagram above, which is a typical diagram when we learn about seasons in school, due to the axis tilt of our planet with respect to its orbit around the Sun, sometimes our Earth’s northern axis is tilted to the Sun and sometimes the southern axis is tilted to the Sun.

Another way of expressing is there will be time when our Sun is above or north of the equator and time when it is below or south of the equator. Only during the equinoxes, the Sun is exactly on the equator.

So, the Sun rises northeast from March to September, and then on today, the Sun will pass by the equator and heads south. After that from September to March next year, our Sun will rise southeast and set southwest. Then on the Vernal or Spring Equinox in March, the Sun once again passes by the equator and heads north and the cycle repeats itself.

Astronomy Picture of the Day has a nice photo showing the different position of the Sun for the northern hemisphere. Just bear in mind that this photo does not reflect our situation at the equator. At the equator, the “middle band” Sun should be directly overhead (at the zenith), while the solstice Suns (“top band” and “bottom band” Sun) are the same distance above the horizon.

~ by thChieh on September 22, 2008.

Posted in General Astro Talk
Tags: Equinox