Sunday, December 21, 2014

Tonight's Sky for December 21: Winter Solstice and New Moon

Technical problems resolved again . . . hopefully for good . . . back in business!

Today marks two major celestial events with one being a once a year event and the other a once a month (usually) happening.
Today marks the Winter Solstice for us living in the Northern Hemisphere. With the Winter Solstice comes the shortest day of the year and the official start of the winter season.


So, why do we have seasons, anyway?


It's all about Earth's tilt. If the Earth were spinning on its axis with no tilt at all, everyone would be treated to days of identical length every day of the year, with latitudes nearer the equator having longer days than those nearer the poles. However, with the tilt, the angle of the Earth relative to the Sun changes as or planet moves about its orbit. On the Winter Solstice, the Northern Hemisphere is tilted up and away from the Sun. On the Summer Solstice (in June), the Northern Hemisphere will be tilted down toward the Sun. On the equinoxes, the tilt is half way between the solstices.

In practical terms, the Sun has its most Northerly rise/set with highest arc through the sky in summer and the most Southerly rise/set with lowest arc in winter.


Second
, the Moon, second brightest object in the sky, has reached the New Moon phase, which means that it is directly between the Sun and Earth, and thus invisible for us Earthlings as of now.

As for lunar mechanics, the Moon is always half lit. The reason we don't always see it as such is thanks to orientation in relation to us. Right now, with the Moon directly between the Earth and Sun, e can't see any of its lit side.

After today, we will see more of the Moon each night as its lit side turns more toward from us and heads toward first quarter.

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