High School

a) It is 4 PM on the spring equinox. What is the local sidereal time?

b) The local sidereal time is 19:30. When will Vega cross your meridian?

c) You observe a star that has an hour angle of 13 hours (13h) when the local sidereal time is 8:15. What is the star’s right ascension?

d) The Orion Nebula has a declination of about -5.5° and a right ascension of 5h 25m. If you are at latitude 40°N and the local sidereal time is 7:00, approximately where does the Orion Nebula appear in your sky?

Answer :

(a) The vernal equinox is at zero hour on the spring equinox, which occurs around March 20. (360 degrees / 23 hours 56 minutes) The sidereal time advances by around 4 minutes every day.

The time since noon is 8 hours, or 480 minutes, as it is 4pm, or 8 hours after noon. Hence, the local sidereal time is roughly: Sidereal time is equal to 0h plus (480 min x 4 min/min)/60 min/hour Sidereal time is equal to 0 hours and 32 minutes 0h 32m is the sidereal time. (b) Vega will cross the meridian at 19:30 local sidereal time since the local sidereal time is 19:30. (c) The difference between the local sidereal time and the star's right ascension is the hour angle. Hour angle equals Right Ascension minus Local Sidereal Time. Hence, the right ascension can be calculated as: Right ascension = Local sidereal time - Hour angle Plugging in the values given.

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