Lunar and Planetary Elongation Chart for 2021
As I did last year, I am posting a lunar and planetary elongation chart for the coming year. We kept a copy of last year’s chart on the refrigerator for easy reference and found it moderately useful. Feel free to download a copy, without any warranty of accuracy. You will notice that all of the curves begin this year exactly where they ended last year, giving some confidence that the charts are at least self-consistent but please leave a comment if you find errors, etc., have a question, or just want to leave a comment. I have rotated the chart this year and added finer annotation to the date axis making it a little easier to find the proper date.
The earth and other planets revolve around the sun. The moon revolves around the earth, but its phase is determined by its position relative to the sun. The consequence of these motions is that the planets and the moon change position in the earth’s sky relative to the background of distance stars and, more importantly, to the sun. The elongation as plotted on this chart is the angular separation of the object from the sun as seen from earth charted over time.
Here are some things you can determine from the chart:
- The morning and evening “stars” — planets, often bright, that occur in the sky near the sun before sunrise and after sunset.
- When the planets and/or moon are close to each other in the sky; an appulse.
- This is possible because the planets and the moon all move in planes close to the ecliptic plane, the orbital plane of the earth. When they have similar elongation values, they are in the same part of the sky.
- The relative brightness of the planets. (but not the moon)
- The phase of the moon.
Some important features of the chart:
- The horizontal axis plots the elongation; the vertical axis plots the date during the year
- The horizonal center of the plot is zero, the location of the sun. Note annotations at the top of the chart.
- The right half shows western elongations, eg, the morning sky just before sunrise.
- The left half shows eastern elongations, eg. the evening sky just after sunset.
- Phase descriptions of the moon are annotated at the bottom of the chart.
- The moon is Full when its elongation is 180° or equivalently -180°.
- The moon is New when its elongation is 0°
- The first quarter moon occurs at -90° , eg.in the evening sky.
- The last quarter moon occurs at 90°, eg. in the morning sky.
- The marker size shows the relative magnitude (brightness) of the planets (but not the moon).
- The markers are placed every four days along each curve.