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| What's Up Now A Virtual Planisphere |
| This is a Java applet showing what's visible in the sky at a
specific location and time. The view is what you would see if you were lying on the ground
looking up, with your head pointing north. In this position, east is on your left, west on
your right, and south toward your feet. Initially, the latitude is set at 40°N, the
longitude is estimated from your computer's time zone setting, and the time is the current
time on your computer. The applet and its data together are only 62K, so you shouldn't
have to wait long for it to load (unless Comcast slows down). If the applet never starts,
Java might be disabled in your browser (a setting you can change), or you might need to
update your Java. You can read more about this here or go directly to Sun's download site. |
| You can use the little arrows to the right of the date field to step the
date and time in increments you choose. Just highlight the part of the date or time you'd
like to step through. The latitude and longitude are integers, since there's no need for
higher accuracy. Star positions, brightness and color (derived from the B-V index) were taken from the Yale Bright Star Catalog. All the stars in the BSC down to magnitude 5.5 are included. Constellation lines are based on those in Donald Menzel's A Field Guide to the Stars and Planets (Houghton Mifflin, 1964), which I've mentioned elsewhere. The nice thing about Menzel for this is that the star charts include both constellation lines and star labels, making the figures relatively easy to transcribe. Some of the figures had to be altered, however, because they included stars dimmer than 5.5. Planet positions are calculated from orbital elements. Since this doesn't account for perturbations, it's less accurate than VSOP87, but it's also much smaller and faster, and the error amounts to less than a pixel over several decades near the year 2000. I used the elements in the Explanatory Supplement to the Astronomical Almanac (1992) and the method explained in chapter 33 of Jean Meeus's Astronomical Algorithms 2nd ed. (Willmann-Bell, 1998). The calculation of the Moon's position is derived from MINIMOON.PAS in Astronomy on the Personal Computer 2nd ed., by Montenbruck and Pfleger (Springer-Verlag, 1994). Although not used yet, the code also calculates the phase and the position angle of the bright limb, both based on Meeus. Eventually, I'd like to draw the Moon with phases, rather than as a featureless dot. The applet was compiled and tested with version 1.4.2_05-b04 of Sun's JRE for Windows. |
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