DarkSkyMan
August 31st, 2009, 08:20 PM
What is the best or most common way to find your way around the stars?
With longitude and latitude i noticed that there are two types of systems. J2000 and Jnow. Can someone explain this please?
Brett
AstroBob
September 2nd, 2009, 08:33 AM
Hi mate, You should be using Jnow. The J2000 system is for the year 2000, but things slowly creep out of sync because of precession. there is very little difference in the two systems. If you are getting coordinates from a book, check what the epoch is and use that system. Here is a cut and paste from starry night -
Precession of the equinoxes: The slow wobbling of Earth’s axis in a 25,800-year cycle, caused by the gravitational attraction of the Moon on Earth’s equatorial bulge. Precession causes the vernal equinox (and all other points on the ecliptic) to move westward along the ecliptic, slowly changing the equatorial co-ordinate grid.
Bob
glenluceskies
September 6th, 2009, 04:55 PM
The coordinates of all objects in the sky are changing constantly, albeit minutely.
To make it easier to record Right Ascension and Declination coordinates, rather than trying to update them every year, the coordinates are calculated every 25 years (1950, 1975, 2000, 2025). I believe that is how it has been done for some time.
Naturally, RA and Declination for this year (2009) will be slightly off compared to 2000 and a long way off compared to 2025, but we are only talking seconds, or a minute or two. So to locate almost any deep sky object for 99% of the amateur astronomy population, using J2000 in 2009 is quite acceptable. there is no reason why you couldn't locate hundreds or DSO's with J2000 coordinates in the next few years.