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How to use the Constellation Java Applet
Latest version: 8/28/96
Problems viewing the applet? Look at the troubleshooting section below.
About the program
This program shows you the stars and constellations in “Star Chart” format. This stars are plotted according to their RA and Dec (see below). The other usual way stars are displayed are in “Horizon” format, where the stars are plotted according to their NSEW direction and their altitude above the horizon. The disadvantage of Horizon format is that a star’s position depends on the date, time and position on the Earth. For simplicity, I have chosen Star Chart format for this program. For an example of a similar program that uses Horizon format, try StarGazer by firstname.lastname@example.org.
Use the scrollbars at the bottom and right side of the Star Window to look at a different part of the sky. The view might seem strange to you at first, but keep in mind that you are looking at the inside of a sphere of stars! The horizontal scrollbar rotates the sphere while the vertical one shows you the top or bottom of the sphere.
This is a picture of what the applet looks on a Silicon Graphics workstation
You can make the program display a larger or smaller number of stars by pressing the buttons marked “More stars” and “Fewer stars.” Displaying fewer stars makes the program run faster.
If you hold down the mouse button on any part of the Star Window, the program prints out the RA and Dec coordinates (see below) for that point on the sky.
Right Ascension (RA) and Declination (Dec) are the names of the coordinates used to specify the position of an object, such as a star, in the sky. They are very similar to the Earth-based coordinates of longitude and latitude.
Right Ascension is similar to longitude on the Earth, however it is numbered differently. Longitude starts at 0 degrees at the Prime Meridian and goes to 180 degree East or 180 degrees West. Since the Earth is a sphere, 180 E is equal to 180 W. Right Ascension, on the other hand, is measured in hours instead of degrees, for historical reasons. It starts at 0 hours at the “First Point of Aries” and goes all the way around to 24 hours which is at the same place as 0 hours. So a star that has an RA of 12 hours is on the opposite side of the sky from one that is at 0 hours. The tricky part about RA is that the First Point of Aries doesn’t stay still like the Prime Meridian does. As the Earth rotates, the First Point of Aries moves. To figure out where it is at a given time and date, you need to use some trigonometry, which I won’t get into here.
- It says “You must have a Java enabled browser…”
- Only modern browsers like Netscape 2.0+ or Microsoft’s Internet Explorer can display Java as of the time I wrote this. Make sure the browser you use can display Java.
- It takes forever to start up.
- Yeah, I know. There’s nothing I can do about it though. There are 9,000 stars in the catalog and it takes a long time to download info on all of them.
- Any other questions? Email me at email@example.com.