Where is the sun at 6.00pm GMT ?

[jandrew]

I thought that this was simple, but the more I think about it the more complicated it gets.

The next time that I move my wind vane I shall want to be very accurate in lining it up to the North.
I could use the pole star, for North, but that wil be difficult at night.
I could use the sun, for South, at noon (adding 1 hour for DST and another 7.3 minutes to allow for my longitude (1deg49')).

Using the pole star or sun is not easy because the bar that I fix to the wind vane to is at right-angles to those directions and will be 7 metres above ground level (a bit difficult to get the sight-line).

So - can I line up the bar to East-West towards the sun at 6.07pm(GMT)?
Sighting down the bar (or on the shadow) should be very accurate.
That's the bit that I thought would be easy.

BUT ---

Is the sun due West at 6.00pm (GMT and on the meridian) or is it North of West during "summer" and South of West in the "winter"?
Indeed, is the sun due West only at either equinox?

If the sun is North (or could it be South) of West during the summer, what is the adjustment for my location at 53deg46' ?
I would need to use that adjustment because in my part of the world the sun has set before 6pm(GMT) during the "winter" half of the year.
In any event, it more often doesn't appear all day during those months and would be too low in the sky to give a useful shadow.

Who can help?

If all else fails, I could use a compass. What is the current magnetic deviation for my location?

jandrew

[AllyCat]

Hi,

It's not a easy as that. You haven't taken into acount "The Equation Of Time". Due to the Earth's elliptical orbit the sun is overhead at "noon" on only four days each year. On other days it can deviate by up to about 17 minutes of time or over 4 degrees (the worst is in November) so you need to consult Astronomical or Navigational tables for data on the specific date (The Nautical Almanac is probably the reference document).

For other times of day the NA gives exact data throughout the day, but I think the sun will always be due East/West 6 hours before/after it is exactly South).

Currently I believe the magnetic deviation is quite small in the UK (a few degrees) so it may be easier to find magnetic deviation tables for your location (perhaps from the Ordnance Survey). Providing of course that you can trust your compass accuracy WRT local magnetic influences, etc..

Cheers, Alan.

[b.e.wilson]

I use one of the many compass apps on my smart phone to get the magnetic declination at my location. Then I use a good sighting compass to measure the bearing of the wind vane.

[jandrew]

Currently I believe the magnetic deviation is quite small in the UK (a few degrees) so it may be easier to find magnetic deviation tables for your location (perhaps from the Ordnance Survey). Providing of course that you can trust your compass accuracy WRT local magnetic influences, etc..

That is why I would prefer not to use a compass.

The sun is nice and big, it castes a useful shadow. It is the obvious thing to use - so long as I can be coinfident about the times at which it is due East/West.

jandrew

[RayProudfoot]

I used Google Maps to determine that due West was half-way between a chimney stack and the roof edge on a nearby house. That way I wasn't worried about local noon which as you point out can be quite complicated to calculate.

Even if you were 10 degrees out I doubt it would have a major effect on your wind direction accuracy especially if you live in a less than ideal environment. i.e. nearby houses, trees and the anemometer located at less than 33ft.

[jandrew]

I used Google Maps to determine that due West was half-way between a chimney stack and the roof edge on a nearby house.

Hey !!!
I like that idea - must get googling

jandrew

[mcrossley]

By co-incidence (in Manchester) at the moment the Sun will due West at 6:00pm BST (not GMT), and due East at about 8:30am

You can use an on-line tool like this one to check other dates/locations.