To compare what you see on Mars with a map, you need to know which side of the planet you’re looking at. This is given by the longitude of the central meridian (CM) — the Martian longitude of the imaginary line down the center of the planet's disk from pole to pole. It is also helpful to know which of the planet's poles is tipped toward Earth, and whether the planet's axis is straight up and down or tipped clockwise or counterclockwise.

Sky & Telescope's Mars Profiler, which will open in a new browser window, is a JavaScript utility that tells you all this and more for any date and time. It also shows a map of Mars so that you can identify any bright and dark markings you see. (Note: Mars Profiler is compatible with Internet Explorer 5 and higher and Netscape 6 and higher, but it is not compatible with Netscape versions 4.x.)

Here's what Mars Profiler looks like:

The display has four parts. At upper left is the date and time; when the routine opens, it is initialized to the present (as determined from your computer's clock). Change the date and time by entering new values in the corresponding boxes and clicking the dark gray Recalculate button. Or click on the buttons in the next row to step backward or forward in increments of 1 day or 1 hour.

Our Mars Profiler uses Universal Time (UT, essentially the same as Greenwich Mean Time), and beneath the time buttons it shows what we think is the offset between UT and your local time, based on your computer's current settings. When changing the time manually using the Time input box, enter the Universal Time that corresponds to the local time when you will be observing. (Note that this works correctly only if your computer's clock is set properly, including an adjustment for daylight-saving time, if appropriate.)

At upper right is a map of Mars, based on computer graphics by planetary cartographer Ralph Aeschliman, showing the planet's main albedo features (dark and light markings). When the routine opens, south is up, matching the inverted view seen in a Newtonian reflector in the Northern Hemisphere. The central-meridian (CM) longitude is shown at the top, and major bright and dark features are labeled for easy identification. A red circle indicates the area of the planet's surface pointed directly toward Earth.

Below the time buttons and the map are three buttons you can use to change the orientation of the map to match the view in your telescope. "Direct view" puts celestial north up and celestial east to the left. "Inverted view," already mentioned as the default, puts south up and west to the left. "Mirror reversed" puts north up and west to the left, matching the view in most catadioptric (mirror-lens) and refractor telescopes used with a star diagonal in the Northern Hemisphere. The Martian longitude scale appears along the south edge of the map in the direct and mirror-reversed views. Note that as used here, "north," "south," "east," and "west" refer to directions on the sky, not directions on the surface of the red planet itself, where Martian longitude increases to the west.

Mars rotates in the same direction as Earth. So from hour to hour its surface markings move from left to right if your telescope presents a direct view with celestial north up and east to the left. But most astronomical telescopes present a north-up view that is mirror-reversed or a south-up view that isn't; in either case, the red planet's surface markings will appear to move from right to left as the hours go by. You can see this by clicking the "+ 1 hour" button repeatedly.

A Martian day lasts 24 hours 37 minutes. So in one Earth day of 23 hours 56 minutes, Mars doesn't quite make one full turn on its axis. This means that if you look at Mars in a telescope at 24-hour intervals, the planet seems to have rotated "backward" by a small amount. You can see this by clicking the "+ 1 day" button repeatedly.

The bottom part of Mars Profiler's display shows basic data about Mars corresponding to the date and time of the map. At its brightest and closest in late October and early November 2005, Mars will shine at an apparent visual magnitude of –2.3, much brighter than Aldebaran and Capella, the two brightest stars in the vicinity at that time. The red planet will then span an angular diameter of 20", smaller than the 25.1" it reached during its last opposition in August 2003, but still plenty large enough to look impressive in a telescope.

One astronomical unit (a.u.), based on the mean Earth-Sun distance, is 149.598 million kilometers or 92.956 million miles. Elongation refers to Mars's angular displacement from the Sun, and illumination to the planet's phase (100% is full). The position angle (p.a.) of Mars's north pole is measured counterclockwise from celestial north through east. So a p.a. of 10° means the planet's north pole is tipped slightly east of celestial north, and a p.a. of 350° means it is tipped slightly west.

• Launch S&T's Mars Profiler