SkyandTelescope.com's almanac presents basic astronomical data customized for your location, for any date from A.D. 1600 to 2400. The information available for display includes sunrise and sunset times, morning and evening twilight times, moonrise and moonset times, the Moon's phase, a list of naked-eye planets visible in the evening and morning skies, rise and set times for each of these objects, and more.

Getting Started

When you launch the almanac, if first gives you the option of selecting your location from our list of cities, or by pinpointing your locale by latitude, longitude, and time zone. If you chose to select your latitude/longitude, you'll need to know your time zone and its offset from Universal Time. If you would rather set the utility to the nearest large city to you, then click the first red link in the top of the intro page.

Regardless of your selection, the SkyandTelescope.com almanac opens in a wide rectangular pop-up window. (If your browser uses a pop-up blocker, you'll need to turn this feature off or, at the very least, allow pop-ups from SkyandTelescope.com.) Unless you've already entered in your location, the almanac initializes itself to the current date and time based on your computer's clock and sets the latitude and longitude to that of Sky Publishing's offices in Cambridge, Massachusetts.

For those who don't live in Cambridge, MA, and haven't preset the almanac, the first thing you'll need to do is change your location. Use the Country and City pull-down menus to the right of the SkyandTelescope.com logo. Note that if you select USA, the program will pause briefly to populate the City pull-down list with the hundreds of choices available.

Once you've picked the city nearest your location, the almanac sets the corresponding time zone and decides whether daylight-saving time (DST) is in effect. If it is, but the DST checkbox isn't checked, check it yourself by clicking on it. Conversely, if DST is not in effect and the box is checked, uncheck it. The local time should now be correct. If it isn't, you may need to reset your computer's clock.

The Almanac Display

From left to right, the almanac shows a graphic representation of the Moon's phase and then reports the Moon's age (in days since the last new Moon), the Julian Day number (for "official" astronomical timekeeping), your location (country and nearest large city), the current date and time (in Universal Time, which is essentially the same as Greenwich Mean Time), your latitude and longitude, time zone, local date and time, and whether daylight-saving time is in effect.

The Moon's displayed phase is accurate to within a fraction of a day. It is indicated using 28 different symbols, whereas a complete lunar cycle takes 29½ days. Why the discrepancy? Simply to ensure that on the dates of new, first-quarter, full, and last-quarter Moon, the correct symbol is always shown.

Next comes a row of buttons that enable you to step backward or forward in time by 1 hour, 1 day, or 1 week. Below them you'll find the local sunrise and sunset times, moonrise and moonset times, and times when twilight ends (in the evening) and begins (in the morning). To the right are two buttons labeled "Less info" and "More info." Hang tight; we'll discuss those in a moment.

Next you'll find listings of which naked-eye planets, if any, appear in the evening and morning skies, followed by a text box indicating any significant astronomical events that occur on the specified date, such as a meteor shower or a conjunction of the Moon and a bright planet or star.

To change the date and/or time, replace the data in the Date and/or Time input boxes, taking care to maintain the same formats. Then press the Calc button to update the almanac. Clicking Reset restores the almanac to the current date and time. You may also step forward or backward in time by 1 hour, day, or week by clicking the appropriate buttons. Remember to check or uncheck the daylight-saving time box if necessary.

More Info

Press the "More info" button. The almanac window will expand to list detailed information about the Sun, Moon, and five naked-eye planets.

Listed first are the coordinates of right ascension (R.A.) and declination (Dec.), which give the object's position on the celestial sphere for the equinox of date. Next are listed the apparent visual magnitude; angular size in arcseconds (arcminutes for the Sun and Moon); percentage of the disk illuminated by sunlight (phase); and, for the specified location, the local times of the object's rising, transiting (appearing highest in the sky), and setting.

For more information, see the Sun, Moon, and Planets pages of Sky & Telescope magazine.

• Open SkyandTelescope.com's interactive almanac