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Thursday, Apr. 17, 2014

Why astronomers are happiest in winter

Thursday, January 26, 2012

It has been a little over a month since the winter solstice, the beginning of winter, when the sun reached its most southern point of the year and started moving back north.

Since that time the days have been growing longer and the nights shorter, have you noticed?

On Sunday, Jan. 1, there was a total of nine hours and 24 minutes of daylight. On Tuesday, Jan. 31, there will be 10 hours and eight minutes of daylight, an average increase of about two minutes a day.

That means that instead of the sun rising at 7:22 a.m. MST and setting at 4:46 p.m. MST as it did on Jan. 1, on Jan. 31, it will rise at 7:10 a.m. and set at 5:18 p.m. (If you live in Southwest Nebraska. Other locations will vary accordingly).

All that is because of the 231⁄2-degree tilt of Earth's axis. At present we are partially tilted away from direct sunlight. As the days progress we will be tilted more into longer daylight.

While that is great for the average person and those wanting to plant a nice garden, we astronomers are not necessarily happy with the shorter hours of darkness.

The sun may have set at 4:46 p.m. on Jan. 1, but the point that is called "astronomical twilight" when objects can be viewed easily is not until 6:23 p.m.

There are three stages of twilight. The first is called "civil twilight." It is defined as the time when the center of the sun is at least six degrees below the horizon.

During this time, objects can still be seen and only very bright objects in the sky -- like Venus and Jupiter -- can easily be seen. This is also the time when drivers would normally turn on their car headlights.

The second is "nautical twilight." It is when the sun is between six and 12 degrees below the horizon. Objects are not as easily seen, but navigational readings using the horizon and bright stars can still be taken.

The final stage is "astronomical twilight." Then the sun is between 12 to 18 degrees below the horizon and the sky is dark enough that even small, dim objects can be observed easily.

At this time of the year, that is roughly two hours after sunset and two hours before sunrise.

In the summer, when days are longer and nights shorter, the amount of time an astronomer has for active viewing is only from about 10:30 p.m. to 3:30 a.m., or about five hours.

That is not much time for observing the wonders of the night sky.

SKY WATCH: New moon was on Monday, Jan. 23, first quarter will be Monday, Jan. 30.

Venus in the west and Jupiter high in the south are the bright beacons these cold January evenings just after sunset. On Friday, Jan. 27, look near the moon about an hour after sunset for the dim planet Uranus.

The planet will be below and left of the moon, at about the eight o'clock position. Use binoculars to locate it. The pair will be on exact opposite sides of the field of view. On Sunday, Jan. 29, the moon will be next to Jupiter.

NEXT WEEK: More astronomical blathering.


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Vernon Whetstone
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