Sunday, March 20, 2011

Happy Vernal Equinox Day!!

Otherwise known as the start of Spring.

Today at 7:21pm EDT will officially mark the end of Winter and the start of Spring in the Northern Hemisphere. In the Southern Hemisphere, the opposite is true: today marks the end of their summer and the start of their fall.

Astronomically, what does this mean?? The Earth's rotational axis is tilted by ~23.5 degrees with respect to the plane of the "ecliptic" (the apparent path the Sun takes across the sky over a year as seen from Earth). BTW it is precisely this tilt which is the cause of the seasons (NOT due to changes in distance)!! If we now imagine extending the Earth's equator out onto the night sky (the "celestial sphere"), and call this the "celestial equator", the equinoxes mark the 2 times in the year that the ecliptic and the celestial equator intersect. In March, we call this the vernal equinox, and in the fall, we call it the autumnal equinox. The consequences of this intersection are:

1) Today we will have approximately equal hours of daylight and darkness. As we get closer to summer, our hours of sunlight increases until we reach the Summer Solstice (the highest point the Sun will reach in our sky; see diagram). This pattern is symmetrical in that we will again have equal hours of daylight/darkness at the autumnal equinox.

2) On this day, the Sun rises due East and sets due West. Various cultures built structures to keep track of the Sun's motion -- really a calendar! Indeed, one of theories that attempt to explain what Stonehenge is has to do with its ability to predict the equinoxes and the solstices.

For us Northerners, the start of Spring is welcomed as it marks the beginning of warm, sunny weather full of colors as flowers blossom and trees bloom. It also tells us we survived the end of an another cold, snowy winter. For me, it reminds me of the start of the major league baseball season (Go Blue Jays!) and that beach weather is right around the corner!

To learn more, consider attending one of our public star talks at JMU's John C. Wells Planetarium! We have exciting dome shows every Saturday at 2:30 and 3:30, followed by star talks, during the school year.

Shanil Virani