From ce599e4f9f94b4eb00c1b5edb85bce5431ab3df2 Mon Sep 17 00:00:00 2001 From: toma Date: Wed, 25 Nov 2009 17:56:58 +0000 Subject: Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features. BUG:215923 git-svn-id: svn://anonsvn.kde.org/home/kde/branches/trinity/kdeedu@1054174 283d02a7-25f6-0310-bc7c-ecb5cbfe19da --- doc/kstars/sidereal.docbook | 80 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 80 insertions(+) create mode 100644 doc/kstars/sidereal.docbook (limited to 'doc/kstars/sidereal.docbook') diff --git a/doc/kstars/sidereal.docbook b/doc/kstars/sidereal.docbook new file mode 100644 index 00000000..a60ab44d --- /dev/null +++ b/doc/kstars/sidereal.docbook @@ -0,0 +1,80 @@ + + + +Jason +Harris + + +Sidereal Time +Sidereal Time +Hour Angle + + +Sidereal Time literally means star time. +The time we are used to using in our everyday lives is Solar Time. The +fundamental unit of Solar Time is a Day: the time it +takes the Sun to travel 360 degrees around the sky, due to the rotation of the +Earth. Smaller units of Solar Time are just divisions of a Day: + + +1/24 Day = 1 Hour +1/60 Hour = 1 Minute +1/60 Minute = 1 Second + + +However, there is a problem with Solar Time. The Earth does not actually +spin around 360 degrees in one Solar Day. The Earth is in orbit around the +Sun, and over the course of one day, it moves about one Degree along its +orbit (360 degrees/365.25 Days for a full orbit = about one Degree per +Day). So, in 24 hours, the direction toward the Sun changes by about a +Degree. Therefore, the Earth has to spin 361 degrees to make +the Sun look like it has traveled 360 degrees around the Sky. + +In astronomy, we are concerned with how long it takes the Earth to spin +with respect to the fixed stars, not the Sun. So, we would like a +timescale that removes the complication of Earth's orbit around the Sun, +and just focuses on how long it takes the Earth to spin 360 degrees with +respect to the stars. This rotational period is called a Sidereal +Day. On average, it is 4 minutes shorter than a Solar Day, because +of the extra 1 degree the Earth spins in a Solar Day. +Rather than defining a Sidereal Day to be 23 hours, 56 minutes, we define +Sidereal Hours, Minutes and Seconds that are the same fraction of a Day as +their Solar counterparts. Therefore, one Solar Second = 1.00278 Sidereal +Seconds. + +The Sidereal Time is useful for determining where the stars are at any +given time. Sidereal Time divides one full spin of the Earth into 24 +Sidereal Hours; similarly, the map of the sky is divided into 24 Hours +of Right Ascension. This is no +coincidence; Local Sidereal Time (LST) indicates the Right +Ascension on the sky that is currently crossing the Local Meridian. So, if a star has a Right +Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More +generally, the difference between an object's RA and the Local +Sidereal Time tells you how far from the Meridian the object is. For example, +the same object at LST=06:32:24 (one Sidereal Hour later), will be one Hour of +Right Ascension west of your meridian, which is 15 degrees. This angular +distance from the meridian is called the object's Hour Angle. + + + +The Local Sidereal Time is displayed by &kstars; in the Time Info +Box, with the label ST (you have to +unshade the box by double-clicking it in order to see the +sidereal time). Note that the changing sidereal seconds are not synchronized +with the changing Local Time and Universal Time seconds. In fact, if you watch +the clocks for a while, you will notice that the Sidereal seconds really are +slightly shorter than the LT and UT seconds. + +Point to the Zenith (press Z +or select Zenith from the +Pointing +menu). The Zenith is the point on the sky where you are looking straight +up from the ground, and it is a point on your Local Meridian. Note the Right Ascension of the +Zenith: it is exactly the same as your Local Sidereal Time. + + + + -- cgit v1.2.1