What's Up Now
A Virtual Planisphere
|Date and time|
|North latitude||East longitude|
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.
Use My Location button uses HTML5 geolocation. The spec requires the
browser to ask your permission to do this, but the program doesn't send that
data anywhere. It stays inside the code running on your machine. You can use the
little arrows to the right of the date field to step the date and time in
increments you choose. The latitude and longitude are integers, since there's no
need for higher accuracy. The edit field for the date accepts a variety of
formats, so don't be overly concerned about typing it exactly the way it's
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.
Deep sky objects (galaxies, star clusters, nebulas) are labeled with numbers
from either the Messier catalog (the ones starting with
M) or the NGC.
The display includes about 30 of the brightest Messier objects. Since Messier
observed from Paris, the southern hemisphere isn't well represented in his
catalog, so I added about the same number of objects from the NGC for the
southern sky. I also added the Double Cluster in Perseus (NGC 869 and 884),
curiously overlooked by Messier.
Planet positions are calculated from orbital elements. The error compared to more rigorous (and much heavier) methods 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). The code also calculates the phase and the position angle of the bright limb, both based on Meeus, although the position angle currently isn't being drawn.
The map is drawn in the polar azimuthal equidistant projection, one of the common projections for planispheres and all-sky maps.
Some other all-sky mapping sites:
- Sky and Telescope's Interactive Sky Chart
- Sky View Café
- John Walker's Your Sky