If you have been watching the sunsets over the past few months, you might have noticed that they have been slowly moving north along the western horizon. At 1:33 p.m. Saturday, the sun will climb back across the celestial equator into the northern sky, marking the Vernal Equinox and the first day of spring.

The tilt of our planet's axis causes the sun to appear to change its path across the sky as the Earth orbits the sun. In the summer, the sun takes a long, high path across the sky, and we get plenty of sunlight. During winter, the sun is low in the sky, and its trek from sunrise to sunset is short. On the first day of spring and fall, the sun is in the middle of the sky and rises and sets due east and west. Because daytime is about equal to nighttime, we call these days equinoxes.

The sun will continue on this northbound journey until the summer solstice on June 21. After lingering at its northernmost point along the horizon, it will begin its journey southward. This journey from north to south and back to north takes 365 days, or one year.

Celebrate the arrival of spring by joining the staff of Carnegie Science Center's Buhl Planetarium and Observatory and members of the Amateur Astronomers Association of Pittsburgh from March 27--28 for "Space Out! Astronomy Weekend."

This two-day celebration showcases the astronomy and space science that happens at the Science Center and Western Pennsylvania throughout the year. During the weekend, you and your family can increase your awareness about astronomy as you check out the latest telescopes and hear about the hottest astronomical news from guest speakers. Hands-on activities are geared for all ages and interest levels. For more information, go to www.carnegiesciencecenter.org.

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.