THE SOUTHWORTH PLANETARIUM

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43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded January 1970

2021-2022: LXXVI

"Mistakes are the portals of discovery."

-James Joyce

THE DAILY ASTRONOMER

Wednesday, February 2, 2022

February 2022 Night Sky Calendar Part I: February's Stars

ORION DUE SOUTH

On February 2, Orion will cross the meridian at 8 p.m.  At this time, this famous constellation will pass due south.    This event affords us the opportunity to explain the all-important "Two Hour Rule."    This simple rule often helps us track stars and constellations throughout the year.    It is predicated on the following facts:

• Stars rise four minutes earlier each day as a consequence of Earth's orbital motion around the Sun.  (Except circumpolar stars.)
• The constellation positions remain consistent throughout the year.   For instance, Orion will always be due south at 8 p.m. on February 2nd for the rest of our lives.     The precessional motion that causes a shift of the stars relative to the vernal equinox is quite slow, about 1 degree every 73 years.  Consequently, its effect over even a couple of centuries is slight.   Proper motion, the shift of stellar positions resulting from their actual movement through the galaxy, is even slower and is negligible over even a few millennia. 

The Two-Hour Rule tells us that a given star's position a month from now will be the same two hours earlier.       As an example, regard Orion.   We said that this constellation will be due south at 8 p.m. on February 2nd.    So, on March 2nd, Orion will be due south at 6 p.m.    On January 2nd, Orion will be due south at 10:00 p.m.   Orion will cross the meridian at midnight on December 2nd and at 2:00 a.m. on November 2nd.     

The Two-Hour Rule also tells us that a star occupies the same position four minutes earlier each day; a half an hour earlier each week and an hour earlier every two weeks.

We can apply this rule to the other prominent constellations currently visible.

LEO THE LION

Leo the Lion will have completely risen by 8:00 p.m. on February 2nd.   Therefore, we can expect to see Leo completely rise by 6:00 p.m. on March 2nd; 10:00 p.m. on January 2nd; midnight on December 2nd.

Meanwhile....

PEGASUS THE WINGED HORSE

Pegasus, the Winged Horse, will start setting at 8:00 p.m. on February 2nd. So, one can expect it to start setting at 7:00 p.m. on February 16th; at 6:00 p.m. on March 2nd.       And, yes, as the vernal equinox point is just to the south of Pegasus within the constellation Pisces the Fish, we can expect Pegasus to vanish into the dusk rather soon.

We complete our star tour in the north, where we will naturally find the Big Dipper, the famous asterism with Ursa Major, the Great Bear.

Although the Big Dipper is circumpolar, its position changes through the night. At 8:00 p.m. on February 2nd, the Big Dipper's handle appears to point directly at the northeastern horizon. It will assume this same position at 6 p.m. on March 2nd and at 10:00 p.m. on January 2nd.

Becoming familiar with the night sky seems a daunting prospect for most because the sky is adorned with thousands of stars, the positions of which change constantly as a consequence of both the planet's rotational and revolutionary motions. However, by remembering the two hour rule, one can keep track of how the locations of stars and constellations change throughout the year.

Tomorrow, we'll post the first part of the event calendar.

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