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Co-ordinate Confusion

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Hi guys

Can someone help i have two issues both surroundin co-ordinates....

My Lattitude is 51.426168 N

My Longtiude is 0.094048 W

I have verified these are correct but....

1] - Every application, stellerium and so on asks for longtiude as....

deg, mins, sec

How does 0.094948 translate to deg, mins sec? in this case im using polarscope..

2] - This brings me on to RA, again deg, mins, sec. I get DEC and i get RA is how far past an object you are, but where does 0:0:0 begin.

If i go into my garden and look at Rigel it will move anti-clockwise from my point of view so if i stand there and its moved 1 deg to the left and took an hour then its RA 1:1:0 but thats only if rigel was the starting point, how does some 300 miles west of me know where it started......

Am i still making any sense.....

It drives me crazy as an enginneer from the 21st century that i cant figure out what 16th Century sailors seemed to know at a glance


On a side note:

Spoke with Martin at FLO this morning, looks like the beeb has done some good after all with their show apparently he's swamped.

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the .094 deg longitude is about .1 so that would be about 0 deg, 6 mins, 0 secs

oh no - sorry - lattitude is in hours mins & secs so .1 would be errrr - I dont know:o, just put zero, I dont think it makes much difference

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Hi Mav

Your correct co-ordinates are

51° 25' 34" N

0° 5' 38.5" W

RA and Dec are measured from fixed imaginary lines imprinted on the sky and move with the sky, not be confused with terestrial lat/long which are imprinted on the earth and move with it.


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To turn the decimals into minutes and seconds simply multiply the digits to the right of the decimal point by 60.

Thus 0.42168 x 60 = 25.3008 minutes

then do the same again to find the seconds

Thus 0.3008 x 60 = 18 seconds

So your latitude will be 51 degrees 25 minutes 18 seconds North

The longitude calculation works in exactly the same way.

0.094048 x 60 = 5.64288 minutes

0.64288 x 60 = 38.6 seconds

So your longitude is 000 degrees 05 minutes 38.6 seconds West

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RA and Dec are measured from fixed imaginary lines imprinted on the sky


The background on this, if your interested, is that Dec is measured relative to the equator and RA is measured relative to an imaginary point known as "the first point of Aries". It was, at some point in time, where the Sun cross the equator going North, but because of precession etc it's moved a bit now. A useful rough thing to remember though is that at the spring equinox, the Sun is a 0h RA 0d Dec (and moving North).

Your point on "what happens for someone 300miles away" brings in HA, or Hour Angle. Hour Angle is kind of like a local RA, and it measures how far away from your local meridian an object is. So that depends on how far round the world you are. Everybody in the world sees a given object (Rigel, for example) at a single fixed RA, but they all see it at a different HA at any given time.

To turn degrees in to H:M:S, you also need to remember there are 15 degrees in a hour. Which "conveniently" means that a second of arc (RA) is different from an arcsecond (dec). Ah, don't you just hate those 16th century sailors :)

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