declination

[Ram Iyer]

can someone explain how data of declination of a star/planet can be used in locating the object in the sky.

I am ussed to azimuth and latitude.

[Blue Skies]

Assuming you have the coordinates of the planet in RA and Dec, you also need some star charts in front of you (either paper or electronic, either will do.) RA is celestial longitude, fixed with respect to the stars, and Dec is celestial latitude, running from the celestial equator to the poles. The coordinates for the planet should point to where it is on a particular night (or near during a particular week.) Hopefully once you have that location on the charts you can see which constellation it is in, and hopefully you can recognise that constellation in the sky. If you've already done some work in learning the shapes of the constellations you'll quickly be able to pick out the 'star' out of place. If not, take the star charts outside with you, spend some time working out which stars should be there and where the planet should be in relation to them.

Perhaps you can tell us which planet you are trying to find? The ancient five (Mercury, Venus, Mars, Jupiter & Saturn) are all relatively easy as they bright, but Uranus and Neptune require star charts and a bit of work starhopping to be sure you're in the right place.

[Ram Iyer]

for example, Saturn on 20 Mar will have RA 23 48 36 and DEC -02 01 00

how can one use this data to locate the planet for example at 9pm Melbourne

There is no problem locating the planet or the constellation (by star chart or azimuth/Latitude) but the query is about how to use declination data

[Wavytone]

Azimuth and altitude, I suggest, are what you are thinking of.

http://en.wikipedia.org/wiki/Celestial_coordinate_system

The trouble is the sky appears to rotate thanks to the earths rotation, so azimuth and altitude of an object are constantly changing; can't draw maps based on that. Add in the yearly orbit around teh sun and it gets more complicated.

So, to map the sky, RA and dec are the celestial equivalents of terrestrial longitude and latitude, based on a celestial north and south pole, an imaginary equator.

If you know the RA and dec of an object, you can find it on a map. Together with your position (lat, long), the date and time, you can calculate its azimuth and altitude which will tell you where it is.

And yes it does involve some maths.

[Blue Skies]

I'm not sure what you're trying to ask, then. Can be more specific about what you consider to be "declination data"? Did Wavytone's response above help?

[Ram Iyer]

the link has been useful and answers my query to some extent

thanks

[astro744]

Forgive me but I believe your co-ordinates are in error. The co-ordinates for Saturn on the evening of Saturday, March 20, 2010 are RA=12h 09m and DEC=+1deg 47m (source SkyTools3Pro & Stellarium).

Look at declination this way: face due south and look at the south celestial pole (SCP). It is a point in the sky 37deg 50m above the south horizon (latitude of Melbourne). Point to this location with one arm. The celestial equator (declination zero degrees) is 90 deg away from the SCP due north. (90-38=52deg above the north horizon) The celestial equator arcs east to west through a point 52deg above the north horizon. Because Saturn has a DEC of +1deg 47min it is close to zero which means it will rise ever so slightly north of due east. However, due to local horizon variations, ie mountains and trees you may not see it for a good half hour after that slightly more north of east and a little higher.

THE RA is the longitude of a celestial object and to work out the position in the sky you need another parameter and that is Local Sidereal Time (LST). If you have a star chart with co-ordinates you can plot the position of Saturn on the chart and you see what is nearby and match it up with what is in the sky. The LST comes in handy to work out where a given RA is. Stated simply the LST = the RA of an object on the north south axis from the northern horizon to the SCP. The current LST = 19h 00m at 8h 13m Australian Eastern Daylight Time (AEDT). In 5 hours it will be 00h 00m LST and 1:13pm here. Another 8 hours approx (9:13pm) and the LST will be 8h 00m. Saturn is 12h 09m so it is 4h 09m behind. ie. at 1:22am it will be due north and 50 degrees high (50-1d47m approx).

Hope this helps.

[astro744]

Current LST at 19:35 AEDT is 6:22. Saturn rise was 10 minutes ago and its RA is 12h 09m, DEC is +1deg, 47m. Saturn will transit due north in under 6 hours at an altitude of 90 - your latitude - 1deg 47m.