Equinox Sky Show 3/20/10

[erugs]

Just received an email from NASA that says "When the sun sets on Saturday, March 20th, a special kind of night will fall across the Earth."

First the crescent moon will be seen in the western sky. After twilight, in the darkened sky, the Pleiades will appear very close to the moon.

So bring your binoculars and hope for a clear sky.

 

[sardog1]

Warning: Minor thread 'jack commencing.

Why is the azimuth to the sun at sunrise at my location 90 degrees today (i.e., a day early) and yesterday in Anchorage (two days early)? If you check the links, you'll see that it is moving northward enough that it is not 90 tomorrow at either location.

I'm asking because I don't know why, and I'm starting to feel a little Rainman/Monk-ish about the whole thing. Please hurry with the right answer.

 

[Nessmuk]

Warning: Minor thread 'jack commencing.

Why is the azimuth to the sun at sunrise at my location 90 degrees today (i.e., a day early) and yesterday in Anchorage (two days early)? If you check the links, you'll see that it is moving northward enough that it is not 90 tomorrow at either location.

Two possible reasons... The earth is not a perfect sphere, but approximates an oblate spheroid, a shape flattened at the poles. That could also explain the latitude dependence.

The other thought is refraction. The sun actually appears to set some minutes after it would if it were not for the atmosphere bending its light around the curvature. The one day difference at different latitudes could be approximation error, or simply due to the time (longitude) difference between you and Anchorage in calculation of when sunrise actually occurs.

 

[jniehof]

The other thought is refraction.

That's definitely part of it; note that sunrise/sunset are listed as altitude -0.8 degrees.

 

[Nessmuk]

refraction

Here's the paper explaining answer.

 

[sardog1]

Here's the paper explaining answer.

Far freakin' out! Thanks! Now, if just I hadn't stared at the sun for so long today . . .