Spring in the Northern hemisphere is coming! And for many of us, that is welcome news indeed. The first day of Spring is March 20, 2003. But what does this day mean from an astronomical point of view?

What does the first day of spring mean to you? Do images of blooming flowers fill your head? Or maybe it's time for a good old spring-cleaning as the days get longer and the weather gets warmer. Spring means these things to astronomers, but it means something else as well.

The first official day of spring in the northern hemisphere is marked by the vernal equinox ("vernal" means "spring,"). In the southern hemisphere, March 20 is the first day of autumn - the autumnal equinox. For the Northern hemisphere, the autumnal equinox will occur on September 23, 2003.

The equinoxes occur when the Sun crosses over the Earth's equator. What I mean is this: using our imaginations, extend the Earth's equator out into space. This projection of the equator is called the Celestial Equator. Now, even though the Earth moves around the Sun, it APPEARS that the Sun is moving around the Earth since we cannot feel our movement through space. Since the Earth's axis is slightly tilted with respect to the plane of our orbit about the Sun, the Sun's apparent path (called the Ecliptic) is likewise tilted with respect to the Celestial Equator. There are only two places where the Sun's path crosses the Celestial Equator (crossing over the Earth's equator, in other words), and those two moments when that occurs are called the equinoxes.

The sizes and relative distance of the Earth and Sun are not to scale.

At the time of the equinoxes, the Sun rises directly in the east and sets exactly in the west. Day and night are nearly equal in length - 12 hours of daylight and 12 hours of night - at all locations around the world. If you were standing on the Earth's equator, the Sun would pass directly overhead at noon.

Astronomers use the vernal equinox as a key reference in determining the positions of stars on the sky. To determine these positions, it is convenient to imagine all the stars are fixed on a huge sphere centered on the Earth. Recall that I said the projection of Earth's equator outward is called the Celestial Equator. The vernal equinox serves as the 0 degree longitude point for the Celestial Sphere.

For more information on the vernal equinox and a pretty picture or two, see:

APOD inspiring image of the Sun rising exactly along the east-west line

"Spring is Here!"

"Spring Begins on March 20th...But Why?"

Our thanks this week to Dr. Beth Brown. Dr. Brown is an astrophysicist in the National Space Science Data Center at NASA Goddard Space Flight Center. She is involved with data acquisition and archiving, and her research
focuses on the x-ray emission from elliptical galaxies.