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Explanation of the Planetary and Lunar Elongation Calendar

General Comments

An astronomical calendar is a chart of the positions of the various astronomical objects over the course of a year. There are many forms for these calendars; this one uses the solar elongation, the angular distance from the Sun along the ecliptic, as the basis for the relative positions of the Sun, Moon and planets.

Sample Planetary and Lunar Elongation Calendar

The image above is a (low-quality) screen capture of a part of the calendar for the year 2000, northern hemisphere view, in color. In the northern hemisphere, the Sun, Moon and planets rise in the east, climb into the southern sky, and set in the west. Thus, since a northern hemisphere observer faces south to see these objects at their highest, it is natural to put east on the left and west on the right. In the southern hemisphere, the objects rise in the east, climb into the northern sky, and set in the west. Thus, the southern hemisphere view has east on the right and west on the left.

Each day is represented by a single horizontal line on the chart, which reduces the three-dimensional sky to one dimension – solar elongation along the ecliptic. Remember that the ecliptic is the imaginary line of the (apparent) orbit of the Sun around the Earth. The Sun, Moon and the naked-eye planets shown here (Mercury to Saturn), are always on or near the ecliptic, so this chart eliminates their northern or southern angular distance (ecliptical latitude) from it. It also eliminates linear distance from the Earth. By eliminating these two extra dimensions, the chart can show the relative positions of the objects along one line.

The solid horizontal lines in the calendar are the first day of each month. The dotted horizontal lines are Sundays; their dates are shown as small numbers next to the month abbreviations.

The calendar is returned as a PDF file.

The calendar is meant to be printed. If you are going to print it on a black and white printer, use the grayscale option. The color version will print poorly on a black and white printer. In any case, make sure that you are printing at the highest resolution possible for your printer.

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Measurement Scales (A)

There are two scales at the top of the calendar. The lower one is in degrees, the upper in hours. These are equivalent ways of measuring the angular distance from the sun, with 1 hour = 15°. When the Sun is on the western horizon (at sunset), the objects east of the Sun are visible. Thus they are in the evening sky. An object at 90° east elongation would be south; an object at 180° would be rising in the east.

When the Sun is on the eastern horizon (at sunrise), the objects west of the Sun are visible. Thus, they are in the morning sky. An object at 90° west elongation would be south; an object at 180° would be setting in the west.

You can also determine the angular distance on the ecliptic between any two objects on the chart by subtraction. For example, in the sample calendar on January 1st, Saturn is about 15° east of Jupiter. Thus, Saturn rose about 1 hour later than Jupiter that day.

Objects 180° east or 180° west (which are the same point) are said to be in opposition to the Sun, and they climb to their highest point in the sky around midnight. Objects at 0° are in conjunction with the Sun and cannot be seen. For the Moon, four elongations are given special names: 0° is New Moon, 90° east is First Quarter, 180° west or east is Full Moon, and 90° west is Last Quarter.

You can estimate angular distances using your hand. When your arm is fully extended, the width of your thumb is about 2°, the width of your fist is about 10°, and the distance between the tip of your thumb and the tip of your index finger (thumb and finger fully separated) is about 15°.

The hour scale can be used in another way. If you know the position of the Sun, then an object which is a certain number of hours after the Sun will be in approximately the same position that number of hours later, and an object which is a certain number of hours before the Sun will have been in approximately the same position that number of hours earlier. For example, in the sample calendar on January 1st, Mars is about 3 hours after the Sun. Thus, Mars set about 3 hours after the Sun on that day. Also, Venus is about 2.5 hours before the Sun. Thus, it rose about 2.5 hours before the Sun on that day.

You can get the local time of sunrise and sunset from an almanac, your local newspaper, by observation, or other methods. Once you know the position of the Sun at any time of the day, use the chart to determine the positions of the other objects.

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Background Constellation Bands (B)

The constellation bands show the positions of the ecliptical constellations. The abbreviation for the constellation appears in the band just outside the chart. The constellations, their abbreviations and their English names are:

Traditionally, each of the original twelve ecliptical constellations was assumed to include exactly 30° of ecliptical longitude, so that the sky was divided equally among them. This program insteads uses the actual boundaries as set by the International Astronomical Union (IAU), and so this information can be used to locate objects visually once you know the constellations. Note that Ophiuchus is included in the table because the ecliptic actually passes through this constellation just north of Scorpius. The combined Scorpius - Ophiuchus band has the abbreviation Sco-Oph to show their connection. For more information about the ecliptic (zodiacal) constellations, see my tutorial: Learn the Zodiac (Ecliptic Constellations)!.

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Sun's Position (C)

Since this is a solar elongation calendar, the Sun is always at the center of the chart. It is surrounded on either side by a band 10° wide. Objects in this band are too close to the Sun to be seen; they are lost in solar glare.

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Planet and Moon Tracks (D)

The planets and the Moon have each been assigned a certain kind of line.

The colors only apply to the color version. The lines are gray in the grayscale version.

The object's position is computed for 0 hours UTC for each day of the year and these positions are connected by the assigned line. Thus, the wavy solid line down the center of the chart shows the varying elongation of Mercury during the year. You will note that Mercury always stays near the Sun, and, of the planets, has the fastest motion relative to the Sun. The tracks of the other planets are produced in a similar way. Also note that the Moon moves eastward along the ecliptic completing one circuit each month.

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Last modified: Saturday, 09-May-2020 16:29:36 EDT