Lunar and Planetary Elongation Chart for 2018

{click the chart for full size view and to download}

Lunar and Planetary Elongation Chart for 2018

Several years ago our family started using Guy Ottewell’s Astronomical Calendar to guide our skywatching.  It was a wonderful and extensive mix of stories, diagrams, descriptions, and charts along with a textual calendar of events.  The last printing was for 2016;  we have not found  an adequate replacement.  I was looking through the 2016 Calendar in December and discovered the a chart of lunar, planetary, and stellar elongation for the year.  Although I had seen it before, I hadn’t appreciated the amount of information that I could get from it.  I decided to make that chart for my own use; I finally got around to doing it and decided to post it here.  Feel free to download a copy, without any warranty of accuracy; please leave a comment if you find errors, etc., have a question, or just want to leave a comment.

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 vertical axis plots the elongation; the horizontal axis plots the date during 2018
    • The vertical center of the plot is zero, the location of the sun.  Note annotations on left side.
      • The upper half shows western elongations, eg, the morning sky just before sunrise.
      • The lower half shows eastern elongations, eg. the evening sky just after sunset.
    • Phase descriptions of the moon are annotated on the right side 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° , 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.

The data for the chart was calculated once a day at 00:00 UTC with a Python script using the PyEphem package.  The data was then imported into IGOR Pro to be plotted.