Hit the dec!

Bad Astronomy
The entire universe in blog form
Dec. 21 2007 2:30 PM

Hit the dec!

On December 22 at 06:08 UT (01:08 Eastern US time) the Sun will reach the point in the sky called the winter solstice. If you go out at the same time every day and note where the Sun is, it's highest in the summer and lowest in the winter, with the difference being about 47 degrees (twice the Earth's tilt). Once the Sun hits its lowest point in the winter (and no apologies to the southern hemispherite BABloggees), it starts to creep back upwards, and the days start to get longer. More or less.

Anyway, we measure positions on the sky using Right Ascension (or RA) and declination, which correspond to longitude and latitude on Earth. At the winter solstice, the Sun has its lowest declination (-23 or so degrees). Why am I telling you this? To explain the lame title of this post. Duh.

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At the moment of the solstice, the Sun's RA will be 18 hours, corresponding to 270 degrees. We measure RA by starting at the vernal equinox, and call that 0 hours. It's been nine months since then, so the sun has moved 270 degrees around the sky (in reality, the Earth has moved 270 degrees around its orbit, but we're standing on Earth, and we make the rules). So the coordinates of the Sun will be 18 hours RA and -23 degrees dec.

If you are attending any parties for the season, drop that into the conversation. You may think it'll sound nerdy and dumb, but hey! Sometimes these things work out pretty well.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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