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Problem |
Equatorial mounts must be accurately aligned with the celestial pole. In order to position a tripod, or construct a permanent pier, you must determine the direction of true north (or true south in the southern hemisphere) within a few degrees before getting started. Determining true north (or south) using a compass or transit requires knowledge of local magnetic deviation, and the results are often dead wrong.
These instructions assume the telescope resides in the northern hemisphere. If you're in the southern hemisphere, substitute south for north.
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Solution |
The sun's shadow points due north/south when the sun crosses the meridian - at the time of transit. So, if you know the time of day when the Sun transits, you can use a plumb bob (or any weight at the end of a string, or the edge of a vertical building) to cast a shadow that defines true north.
TheSky can be used to determine the transit time of the sun for any day. |
First, it is imperative that you accurately set
your location on Earth in TheSky. If this is a permanent installation
and cannot be changed after the fact I cannot stress this enough. A
simple error in location or time (i.e. not using daylight savings
properly) will introduce up to 1 hour of error in time and that equals
15 degrees of error! After properly setting the location and daylight
option enter the day for which you will be
marking the north/south line cast by Sun's shadow. Now regardless of what time
it is now- TheSky's reported transit time is accurate for the entire
day.
How to accurately set the PC clock <--Click here for details
Once you have the correct date and location set in TheSky, click Edit | Find Sun and then click Find. TheSky's Object Information dialog reports the time that the sun transits for the current date. As long as the date is correct the time will not matter since the Transit time is the same time and only one time for that day. Clear? See below.
Edit | Find Sun
Press Find
The will compute the Transit Time, Rise Time and Set Time for the Sun on any given day. That information is displayed on the object information dialog as shown below.
Check the transit time
of the sun for any given day
Sun's transit time for December 29, 2005
Time Dialog
Set TheSky's time to the sun's
transit time and now look South
The computed transit time for the sun on December 29, 2005 is 12:03 PM. As a sanity check, enter 12:03 PM in TheSky and verify that the sun is on the meridian at this time! A good reality check!
When the sun crosses the meridian (defined as the transit time), it casts a shadow that defines a perfect north/south line. Hold or hang a weighted string (or a plumb bob) to create a vertical obstruction and note the orientation of the shadow. Marking this line with a string affixed at two points or scribed on concrete with chalk makes for an exact center line to orient the pier or tripod. Great for daytime aligning in a portable situation by the way.
TheSky Display December 29, 2005
Note Transit Time and position of the Sun
Using a weighted string or plumb bob to cast a shadow, and then mark the line of the shadow at two points separated by several feet to define the north-south line for your site.
A vertical string or other casts a north/south shadow
Close to NOON on any clear day
But not Necessary 12:00 sharp!!!!!!!
Check transit time in TheSky!
If the mount is already on the pier, the above north/south line or shadow should bisect (splitting in half) the mount nearly perfectly. This technique also works in the field for portable setups, and is more reliable than using magnetic deviation with a compass, or even using a GPS.
Now, just mark the shadow that is cast with two points along the line. This accurate line allows you to orient the pier or tripod quite easily.
Aside: The sun's transit time changes slightly each day, and this is why you see the Sun's Analemma when you plot the position of the Sun every day at noon for say 1 year. This apparent daily motion of the sun is caused by the Earth's orbit being elliptical (not a perfect circle) and the 23.5 degree tilt of the Earth's axis relative to the Sun (the plane of the ecliptic). Try it in TheSky. Just choose a 5 day time skip at 12:00 noon and watch the show!
Sun's Analemma
The sun's Analemma.
NOTE: The top of the analemma is June 21st the first day of summer, the bottom December 21st the first day of Winter, and the two middle points are the Equinoxes (Equal night and day) found on ~March 21, and ~September 21.
