Morsel: a scarcity of planets

Callum Potter posed a question in his Sky Notes for 2011 April & May (J.Br Astron.Assoc. 121, 2, 2011):

"As I was researching these Sky Notes, the dearth of planets to observe was quite surprising. Only Saturn is well positioned, and I started to wonder how often the absence of many of the planets arises."

If we characterise the "dearth" as no planets lying between Eastern Elongations (from the Sun) between 18° (best astronomical twilight end) and 180° (midnight meridian) then it turns out not to be at all rare.

If we make the test slightly more strict: no planets between the Sun and 180° then it is slightly rarer and here is the pattern for about 2 centuries around the present:

Start JDStart dateEnd JDEnd dateNo of days
24207041915-Jul-2524207171915-Aug-0714
24306451942-Oct-1224306811942-Nov-1737
24309951943-Sep-2724310391943-Nov-1045
24317311945-Oct-0224317311945-Oct-021
24317981945-Dec-0824317981945-Dec-081
24346981953-Nov-1624347261953-Dec-1429
24362341958-Jan-3024362341958-Jan-301
24369361960-Jan-0224369611960-Jan-2726
24435011977-Dec-2324435011977-Dec-231
24438961979-Jan-2224438991979-Jan-254
24450031982-Feb-0224450601982-Mar-3158
24457011984-Jan-0124457691984-Mar-0969
24458141984-Apr-2324458241984-May-0311
24473271988-Jun-1424473331988-Jun-207
24486531992-Jan-3124486651992-Feb-1213
24502471996-Jun-1224502691996-Jul-0423
24509481998-May-1424509751998-Jun-1028
24590332020-Jul-0224590452020-Jul-1413
24597232022-May-2324597782022-Jul-1756
24604512024-May-2024604672024-Jun-0517
24649622036-Sep-2524649852036-Oct-1824
24737652060-Nov-0124737992060-Dec-0535
24760002066-Dec-1524760062066-Dec-217
24800482078-Jan-1424800512078-Jan-174
24824432084-Aug-0524824502084-Aug-128
24964572122-Dec-1924964882123-Jan-1932

This was computed with a short Java program using Holst.jar, which is an implementation of the VSOP87 algorithms for major planet positions, accurate to 1 arc second. This JAR file is the basis of our "What's observable" applet. By entering the Julian Dates from the table above into that applet you can see what happens on each occasion. The next occasion, in 2020, looks interesting for the proximity of Jupiter and Pluto, with Saturn not far away, but unfortunately for us in the UK it happens near the lowest point on the ecliptic.

Section Director
Steve Harvey

Section Director, Steve Harvey

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