Which way's west?

[jdg600]

I've seen it stated several times that you can determine which direction is west by switching off the tracking (if your scope has it) and watching for the direction in which ojects drift out of the field of view. Whilst sketching Mizar and Alcor yesterday morning I noticed that, as the Plough was in the north, Mizar appeared to be to the east of Alcor in the sky but, using the above method, drifted out of the field view before Alcor and so appeared to be to the west of it in the eyepiece. I'm therefore confused as to what convention to use in marking up my sketches.

John

[Jiggy 67]

Ouch!!....feel a headache coming on trying to visualise this conundrum!!! .....might be worth posting in the Sketch section

[Qualia]

Polar align your mount and switch off any tracking motors.

Skew your scope to a relatively bright star away from Polaris. For the sake of brevity, I usually go for something like Epsilon Cassiopeiae.

Place this or any other star - other than Polaris - in the centre of your EP's field of view and let it drift. Where it exits is by definition your west, resting at 270º.

Now, looking through your eyepiece (you may need the finderscope to help a little), nudge or move your OTA back towards Polaris.

Your EP's north is where that star enters your EP field-of-view and is by definition at 0º. To make sure you've got it right, the angle distance from north to west on your sketch pad should look like 90º.

If you are using a reflector your north is counter clockwise from your west. If I've got the mental picture right - stars will drift toward the left.

If you're using a refractor, then west will be clockwise from your north. That is, stars will drift toward the right.

Now that west (270º) and north (0º) are known, east (90º) and south (180º) should be easy to work out.

For sketches, two cardinal points ought to be included, so that others can appreciate your art-work observation relative to their own position.

Hope that helps and if it doesn't work out right, please let me know! It's the method I'm using and I'd hate to think my own sketches are erroneous.

[MorningMajor]

West? It's where the sun goes down Which of course, is no use to man nor beast in the middle of the night!!

[brantuk]

Can't you just use the handset to give you the co-ords at start of the session and again at the end of the sketching? Then use Stellarium or charts to check which part of the sky you've tracked through during the sketch?

Or just run Stellarium back to that time of night and monitor where your object was during the sketch - then you only need the time of start and end.

[jdg600]

Thanks for the info Qualia, I'll try that next time I go out but I'm not sure it fully answers my question. What I'm trying to say is that when Mizar is in the north it is moving towards the east and that is the direction in which it drifts out of the eyepiece. I've attached a sketch to try and show what I mean.

[acey]

Directions on the sky can be a bit confusing because they go opposite to directions on the ground. That's because we look at the surface of the Earth from the outside but see the "celestial sphere" from the inside. So when north is at the top on a star map west is on the right, not left.

You also need to be sure what sort of view you're getting in the telescope: a Newtonian (with 2 mirror reflections) gives a rotated (inverted) view, but not left-right reflected. Wherever stars are leaving the field is west, and this is "3 o'clock" with respect to north. A refractor with mirror diagonal gives a left-right reversed view. Where stars leave the field is west, but because of the mirror reversal you now have west at 9 o'clock.

The other thing to consider is that objects on the sky rotate around the north pole, and are moving west all the time with respect to this celestial north, but are going in a circle with respect to the ground. An object can appear to move right-to-left or left-to-right on the sky, depending which side of Polaris it's on ("above" or "below").

If the Plough appears to be "below" Polaris on the sky then the Plough will seem to be moving right, apparently towards terrestrial east. But keep watching long enough and the constellation will circle right round until it is overhead and moving apparently left (while you look north). If you'd been looking at it all the time through a telescope then you would have found your field of view slowly rotating. At any time you observe you will find that Mizar leaves the field of view before Alcor: Mizar is to the west with respect to co-ordinates on the celestial sphere.

The old names for celestial west and east, still often used, are "preceding" and "following", which makes perfect sense at the eyepiece: an object that leaves the field first is "preceding", one that follows afterwards is "following". This nomenclature has the advantage of not getting confused with terrestrial terminology.

[Capricorn]

To the left of North. :grin: