06 April 2012

Vernal Equinox, Full Moon, Passover and Easter

As Spring arrives, so do the religious festivals of Passover and Easter. These two festivals change date each year, happening sometime in March or April. These festivals are tied to the phase of the Moon and the Vernal Equinox (the first day of Spring), and hence they change date each year. Tonight's Full Moon is the event that ushers in these festivals. How does the timing of these events work?

The Vernal Equinox is the first day of Spring, the day when the Earth's tilt is aligned to its orbit around the Sun, so the poles of the Earth tilt neither toward the Sun or away from the Sun. On this day (and again in the Fall at the Autumnal Equinox), every place on Earth experiences the same number of hours of Sun above the horizon and Sun below the horizon, hence 'equinox' for equal lengths of day and night.

The first Full Moon following the Vernal Equinox marks the start of the religious festivals in Judaism and Christianity, with Passover taking place on the day of the Full Moon (today), and Easter on the first Sunday following the Full Moon (this Sunday April 8th).

Tonight's Full Moon will be very close to the bright blue star Spica, the brightest star in Virgo, and just to the left of Spica you will have a nice view of the planet Saturn. Enjoy the view!

Image courtesy of Sky & Telescope Magazine.

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