How do I use Viewing Time and Minimum Altitude together?

Posted: 10 Jan, 2008
by: Lang P.

Updated: 14 Apr, 2008
by: Lang P.

Question: How do the Viewing Time limits get taken into account and inter-react with the Ephemeris Time? For example, a search of all Messier objects at least 25° altitude at 12:00 AM today with Viewing Time set to astronomical darkness today doesn't include M 42. M 42 transits at 32° altitude at around 10:00 PM.

Answer: The Ephemeris Date/Time specifies the instant at which altitude and azimuth are calculated and used for the Sky Position test - in your example, the object must be at least 25° altitude at 12:00 AM. The Viewing Times specify a period of time when objects must be above the horizon to be included. So in the example you provided, M42 was eliminated because of its altitude at 12:00 midnight; had the altitude filter NOT been employed, it would have been included because it was above the horizon during your Viewing Times.

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