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Eroica
2008-Feb-10, 05:39 PM
At what time does Betelgeuse cross the meridian for an observer at sea-level in latitude 55 degrees north on 15 February 2008?

According to Starry Night, at about 20:40.

According to two different planispheres I have, at about 20:12.

Which is right, and why so great a discrepancy?

I know planispheres are not particularly accurate, but 28 minutes!

grant hutchison
2008-Feb-10, 05:53 PM
I'd guess the difference arises because you're significantly west of the centre longitude for your local time zone. The planispheres will give you the time of meridian crossing at the centre of your time zone; then the Earth has to rotate a little further for the meridian crossing to take place at your longitude.
So if you have your longitude correctly entered in Starry Night, I'd trust that answer.

Grant Hutchison

hhEb09'1
2008-Feb-10, 06:52 PM
I'd agree with grant. I don't see anything in your OP that takes longitude into account, and that certainly can make a big difference.

PS: my copy of SkyMap says from Dublin (lat 53 20, lon 6 15), it crosses on Feb. 15 at 20:40. From (lat 53 20 lon 0), it crosses at 20:14, so not quite the discrepancy that you're looking for. Hmm, if I change the latitude to 55, then SkyMap still says about 20:14. Close, though.

Is it marked off in fifths or tenths of an hour? :)

Hornblower
2008-Feb-10, 07:11 PM
It should be independent of the latitude. Something does not compute here.

2stepbay
2008-Feb-10, 08:17 PM
It's more about longitude. Further to the east or west from the mid-point of a time zone, the greater the difference from that value derived at the mid-point line. As there are 24 time zones, values can be + or - 30 minutes depending on location within that time zone.

hhEb09'1
2008-Feb-10, 08:31 PM
It should be independent of the latitude. Something does not compute here.D'oh. Yes, crossing the meridian. But that's what I said, even though I actually had to look it up. :)

Tobin Dax
2008-Feb-10, 09:40 PM
As there are 24 time zones, values can be + or - 30 minutes depending on location within that time zone.
That's only to first order. Since timezone boundaries are uneven, it can be more than that.

But, yes, it's what everyone else is saying: It's your longitude. Meridian crossing should shift by about 4 minutes per degree of longitude you are from the central longitude of your timezone. At about 6 deg, that 26 minute difference is just about right.

Eroica
2008-Feb-11, 11:17 AM
I'd guess the difference arises because you're significantly west of the centre longitude for your local time zone. The planispheres will give you the time of meridian crossing at the centre of your time zone; then the Earth has to rotate a little further for the meridian crossing to take place at your longitude.
So if you have your longitude correctly entered in Starry Night, I'd trust that answer.

Grant Hutchison
:doh: D'oh! I forgot all about the longitude.

Thanks, grant and everyone else. When I switch my longitude to 0 degrees on Starry Night I get 20:15 or thereabouts. Problem solved.