We generally think of the vernal equinox as the start of spring (in the northern hemisphere). After surviving through the winter, we begin the longer days and warmer temperatures that lead to summer.

Astronomers also think of the vernal equinox as the beginning of space and time, in a manner of speaking.

Equinox

The vernal equinox occurs on March 20 or 21 of each year, although I'm told it can sometimes occur on the 19th due to happenstances of leap years and such. In astronomical terms, it marks the time when the Sun passes through the celestial equator moving northward. The celestial equator is the projection of Earth's equator onto the sky.

The path that the Sun follows on the sky is called the ecliptic. It represents the plane of Earth's orbit around the Sun, and we take it as the central plane of the solar system.

Note that the other planets in the solar system orbit the Sun on slightly different planes. Mercury's orbit is tilted with respect to the ecliptic the most, at about 7 degrees. Venus' orbital plane tilt is about half of Mercury's at 3.4 degrees. Saturn's orbit tilt is about 2.5 degrees, and the rest are less than 2 degrees.The ecliptic does not truly represent an average of the planets' orbital planes, but it is a practical definition.

Because Earth's rotation axis is tilted about 23 degrees with respect to the ecliptic, we generally see the Sun either above or below the celestial equator. When the Sun is north of the equator, days are longer in the northern hemisphere. And when the Sun is south of the equator, days are longer in the southern hemisphere. As every schoolchild should learn, the tilt of Earth's rotation axis produces the seasons.

The celestial equator and the ecliptic intersect at two points, called the equinoxes. The vernal equinox is the intersection where the Sun is moving from the southern celestial hemisphere to the northern celestial hemisphere. The autumnal equinox is the other one, where the Sun goes from north to south. The equinoxes mark the start of spring / fall, noting that spring in the north is fall in the south and vice-versa.

Space

Coordinates on the sky have traditionally been a simple projection of Earth coordinates. We are familiar with longitude and latitude to mark any location on Earth.

We also mark the positions of the Sun, stars, etc., with longitude and latitude on the sky. However, astronomers call latitude as "declination", and longitude as "right ascension". The ideas are the same, even if the names and definitions are different. This post is not concerned with those definitions, for that try the wikipedia articles.

My concern here is that every coordinate system must have an origin - a place from which all other positions are measured. What point in the sky is special enough to mark such a spot?

The celestial equator is the obvious choice for the center point of declination. It divides the sky into two hemispheres. We give the celestial equator zero degrees declination, just as we give Earth's equator zero degrees latitude.

The two special points along the equator that are easy to define and identify from observations are the equinoxes. The vernal equinox was chosen as the origin of right ascension.

Hence, the point on the sky where the vernal equinox occurs is the zero point of our celestial coordinates.

Time

As the origin of the celestial coordinates, the position of the vernal equinox is very important to measure. Unfortunately, you can't just measure it once and be done with it. It has to be measured every year.

The problem is that Earth's rotation axis is not fixed in space. Like a wobbling top, Earth's rotation axis moves and traces out a cone. This wobble is called "precession". The point on the sky that the rotation axis points toward, the celestial north pole, is thus not fixed with respect to the stars. It traces out a circle on the sky over time. Precession through the entire circle takes about 26 thousand years. We don't generally notice it on one person's lifetime, but if you look at the star positions of the ancient Greeks, you can clearly see the effects of precession.

Another effect is that the position of the vernal equinox changes slowly. This effect is simply called the precession of the equinoxes. Hence, for high accuracy astronomy, you need to measure the position of the equinox every year. In truth, the equinox is shifting continuously, so measuring a position on the sky is not just a matter of where, but also when. When you publish coordinates, you also attach a time to them. You may see "B1950" coordinates, which have been standardized to a 1950 reference frame, as well as "J2000" coordinates that are referenced against the equinox in the year 2000.

Hence the equinox position and time are important. The equinox time is, in this sense, the zero point of the astronomical year.

Taken together, the vernal equinox is the zero point of space and time.

Stop the World, I Want to Get Off!

As you might imagine, this constantly shifting coordinate system would get to be a nuisance. Although modern computers can easily account for the changes between coordinates measured at different times, why bother? Why should astronomers continue to tie their coordinates to the shifting frame of Earth?

The International Celestial Reference System (ICRS) is the solution that astronomers have devised. It is a celestial coordinate system based not on Earth, but upon very distant astronomical objects. A set of 212 radio sources beyond our galaxy (most of which are quasars billions of light-years away) are used to define the orientation of the coordinate system. It closely matches the J2000 coordinate system, but it will not change over time. Read more about ICRS at the USNO website.

Therefore, this post is really a nostalgic one. The vernal equinox was a special point in time, but, as my kids often tell me, that's so last millennium. In the twenty-first century, we've got a universe-based fixed coordinate system.

However, I'll continue to think of the vernal equinox as the astronomical new year. Perhaps I'm just an old dog.