Equinoxes

“On March 20, 2009, at precisely 7:44 am EDT (March 20, 11:44 Universal Time), the Sun will cross directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.” http://www.infoplease.com/spot/riteofspring1.html

I promised several months ago (on the winter solstice) to write about the equinoxes at some point. As today is the vernal equinox, the official beginning of spring, here it is.

The term equinox is Latin for equal night. On the equinoxes (both spring – vernal and fall – autumnal) there are exactly 12 hours of daylight and 12 hours of darkness for every part of the planet from the North Pole through the Equator on south to the South Pole. These are the only two days a year when that is true.

The Sun’s apparent path in the sky, called the Ecliptic, is due to Earth’s orbit around the sun. The concept of the zodiac is related to this. The sun goes through different constellations (again from Earth’s perspective) during the course of a year. The planets roughly follow this path, too, which is why you’ll never find Mars in the far north of the sky (as viewed from the Northern Hemisphere), but you might see it in Gemini, the Twins (a winter constellation), or Sagittarius, the Centaur archer (a summer constellation).

The night sky has been mapped out like an extension of Earth. A projection of Earth’s North Pole into the sky is called the Celestial North Pole; a projection of the Equator is the Celestial Equator, and so forth.

On the equinoxes, the Ecliptic crosses the Celestial Equator. This means that the Sun will rise due East and set due West on those days. Because folks today do not pay much attention to where the Sun rises and sets, many may think that this is usually the case, but that is not true.

From the perspective of the continental United States, the Sun’s path changes in the following ways over the course of a year. On the winter solstice, the Sun rises the farthest south of east that it ever rises, goes up to the lowest altitude in the sky for the year, and quickly sets again in the farthest south of west position that it ever sets. For a position with a latitude of 38 degrees North (where I live), the Sun rises to be about 29 degrees above the Southern horizon at noon on the winter solstice. The Sun, during the next several months, seems to rise a little more north of its position from the previous day, until, on the vernal equinox, it rises due east, gets up to a middling height, and sets due west. From 38 degrees North, the Sun rises to be 52 degrees above the Southern horizon at noon on the equinox. That’s 23.5 degrees higher than it was on the winter solstice, because the Earth is tilted at an angle of 23.5 degrees. Not surprisingly, then, on the summer solstice, when the Sun is the highest that it will be all year, it’s at a height of about 75 degrees when viewed from the northern US.

Having always lived in the continental US, it was a unique experience for me to be able to see the Sun in the north in the middle of the day when I visited Costa Rica. The Sun had always been to my south at noon for my entire life. In the summer in Costa Rica, at a latitude of 9 degrees North, the Sun passes through the zenith (a point directly overhead) during early summer and stays in the northern part of the sky during much of the summer. The tropics of Cancer and Capricorn (at 23.5 degrees North and South latitude) are called that, because the sun passes through the zenith there one time per year. The sun passes through the zenith only on the solstice in June for the tropic of Cancer (its summer solstice) and the solstice in December for the tropic of Capricorn (its summer solstice, our winter solstice). That’s why the tropics are so warm. The Sun rises to be very high in the sky there every day of the year and the length of day does not change very much there over the course of the year.

I’m beginning to realize that I have gotten so used to teaching this with pictures that it is very hard to describe only in words. This all makes a lot more sense when there are pictures to go along with the explanations. I don’t have more time just now, but please feel free to ask questions or make comments, especially with help in making what I’m trying to say more clear for a non-astro person.

Thanks! And happy equinox!