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Establishing a Meridian: (ref. Establishing Meridian List)

In all the ATM books and articles I have come across; and I reckon I've read them all, not one refers to how you peg out the site so that the telescope pier is aligned north-south.

Telescopes on cylindrical columns, with no restrictions on azimuth adjustment don't need an aligned pier, but what about those that do? If you only have a few degrees azimuth adjustment to play with, getting the pier correctly aligned is an important consideration. And yet again, not once have I heard anything on the subject from the so called "experts". I have witnessed all manner of odd ball methods though.

The favourites are:
i) alignment via a landmark plotted on a 50000 series OS map (but never correction from grid north to true north).
ii) magnetic compass - with possible allowance for magnetic deviation.
iii)sighting Polaris at upper or lower culmination.

They are all a ridiculous nonsense.

The method I am about to describe will suffice, and prove the least troublesome and the most accurate. All you need is a large sheet of thick brown paper and some weights, a tripod and a plumb line and bob, a black felt marker pen, a watch set to GMT, an Astronomical Almanac, a yard rule, and pegs and string. You must also have your longitude to the nearest arc minute which may be obtained from the 20000 series Ordnance Survey.

First of all decide precisely where the telescope's pivot is going to go. Having levelled the site, lay the sheet of brown paper as close to the north-south line as you can estimate, and hold it down with the weights. Place the tripod across the south end and suspend the plumb bob from the head so it can swing freely, with the bob just clearing the paper. Mark the bob point when it has settled. This will be directly Sample Calculationbelow the telescope's pivot point. (Intersection of the RA/DEC axes).

Calculate the time of local noon from the Almanac. To do this look up the time of solar transit at Greenwich. Correct for your longitude. Remember if you are east of Greenwich local noon occurs earlier, and if you are west of Greenwich local noon occurs later. Convert your longitude from arc to time (based on 1º equals 4 minutes time & 1'arc equals 4 seconds time) and add it to the Greenwich time of solar transit if west or subtract it if east. This is the time the Sun will be exactly due south at your site. Set your watch by the speaking clock or the MSF time signals and correct for BST. It is imperative you make this calculation without any mistakes, especially the longitude correction. Bear in mind the tabulated times of solar transit at Greenwich in the Almanac are in Ephemeris Time, not Universal Time (GMT). You will need to apply a correction DT which is tabulated in the back. (The value of DT is NOT trivial - in 1997 it was 63 seconds!)

This is why I keep banging on about the need for rigour in this hobby. It is no use purblindly following the examples of leading lights within the BAA, almost all of whom boast of their inumeracy and dismiss mathematical arguments as "Mere facts and figures." I know of two semi-professional observatories in England, one set up by Professor Vincio Barocas at Longridge in Lancashire, housing the Wilfred Hall 15-inch f/12 Grubb Astrograph, the other set up in 1935 at Mill Hill to accommodate the 24/18-inch Radcliffe Grubb double refractor, that have noticeably misaligned piers. And yet to listen to the luminaries involved in these fiascoes you'd think they had forgotten more about this aspect of observatory construction than you could possibly even begin to comprehend.

For those of you out there who are firmly convinced that mathematics is irrelevant to a proper comprehension of astronomy, think again. "If you don't know it in numbers, you don't know it at all."

Take care, avoid stupid mistakes, and get someone to check your sums.

 

 

Local Noon by ShadowLocal Noon by Shadow

 

About an hour before local noon begin marking off the end of the plumb line's shadow on the north end of the brown paper using the felt marker pen and noting the time. Do this at regular intervals (say 12 minutes), and mark local noon as well if the Sun isn't clouded over at that precise moment. If it is, carry on until about an hour after local noon, and interpolate the shadow marks. The best way of doing this is by drawing a straight line from the first to the last mark and measuring the intervals of each shadow mark along the line from the first, and listing the distance against the time interval.

Once you have established the noon shadow mark, draw a line from the bob point to the noon mark and extend it across the sheet of paper, both north and south. Then peg out a long line so the string runs contiguous with the line on the paper, several yards beyond each end. This is your local meridian. Do not move it - it is sacrosanct.

This page was created by SimpleText2Html 1.0.2 on 2-Feb-102.

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