What's Up Now

A Virtual Planisphere

Using HTML5's canvas element and some Javascript, this page shows 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.

Apparently your browser doesn't support the HTML5 canvas element. Sorry about that.
Date and time
North latitude East longitude
DSOs     Star names Constellation lines Constellation labels

Initially, the latitude is set at 40°N, the longitude is estimated from your computer's time zone setting (accounting for Daylight Saving Time), and the time is the current time on your computer. The latitude and longitude are integers, since there's no need for higher accuracy. The Use My Location button uses IP geolocation. (Most browsers now require a secure (HTTPS) connection for HTML5 geolocation.)

You can use the little arrows to the right of the date field to step the date and time in increments you choose. The edit field for the date should accept a variety of formats, so you shouldn't have to type the date exactly as shown, although this increases the odds that it'll be recognized.

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), a personal favorite that didn't make the cut in Messier's catalog.

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, the position angle of the bright limb, and the parallactic angle, all based on Meeus and used to draw a properly lit and rotated Moon.

The map is drawn in the polar azimuthal equidistant projection, one of the common projections for planispheres and all-sky maps.

I wrote the first version of this (as a Java applet) in 2006 as a fun, non-trivial exercise. To make it more useful for observation planning, I revised it at the end of 2011, enlarging the panel, adding deep sky objects, and drawing the Moon's phase. The first Javascript/HTML5 canvas version was written in 2013, inspired by a SIGGRAPH talk on the canvas element, not to mention the decline of Java applets. In mid-2014, I added star and constellation names and twilight background colors. At the end of 2020, I improved the star colors and finally got the Moon oriented. In mid-2021 I added 16 star names so that all 57 navigation stars are identified by name (thanks to John Pinto for the suggestion).

The planisphere code has been adapted for use on the status pages of the ATLAS asteroid early warning telescopes on Haleakala and Maunaloa in Hawaii. It's also been adapted by the Bath (U.K.) Astronomers club. Greg Benedis-Grab wrote a blog post on Medium that references this page within the larger context of using visualization coding as a means of understanding conceptual models.

Some other all-sky mapping sites:

There are also a few companies that, for a price, will generate star maps for wall hangings, t-shirts, pillows, coffee mugs and the like.