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 JD | Start date | End JD | End date | No of days |
|---|---|---|---|---|
| 2420704 | 1915-Jul-25 | 2420717 | 1915-Aug-07 | 14 |
| 2430645 | 1942-Oct-12 | 2430681 | 1942-Nov-17 | 37 |
| 2430995 | 1943-Sep-27 | 2431039 | 1943-Nov-10 | 45 |
| 2431731 | 1945-Oct-02 | 2431731 | 1945-Oct-02 | 1 |
| 2431798 | 1945-Dec-08 | 2431798 | 1945-Dec-08 | 1 |
| 2434698 | 1953-Nov-16 | 2434726 | 1953-Dec-14 | 29 |
| 2436234 | 1958-Jan-30 | 2436234 | 1958-Jan-30 | 1 |
| 2436936 | 1960-Jan-02 | 2436961 | 1960-Jan-27 | 26 |
| 2443501 | 1977-Dec-23 | 2443501 | 1977-Dec-23 | 1 |
| 2443896 | 1979-Jan-22 | 2443899 | 1979-Jan-25 | 4 |
| 2445003 | 1982-Feb-02 | 2445060 | 1982-Mar-31 | 58 |
| 2445701 | 1984-Jan-01 | 2445769 | 1984-Mar-09 | 69 |
| 2445814 | 1984-Apr-23 | 2445824 | 1984-May-03 | 11 |
| 2447327 | 1988-Jun-14 | 2447333 | 1988-Jun-20 | 7 |
| 2448653 | 1992-Jan-31 | 2448665 | 1992-Feb-12 | 13 |
| 2450247 | 1996-Jun-12 | 2450269 | 1996-Jul-04 | 23 |
| 2450948 | 1998-May-14 | 2450975 | 1998-Jun-10 | 28 |
| 2459033 | 2020-Jul-02 | 2459045 | 2020-Jul-14 | 13 |
| 2459723 | 2022-May-23 | 2459778 | 2022-Jul-17 | 56 |
| 2460451 | 2024-May-20 | 2460467 | 2024-Jun-05 | 17 |
| 2464962 | 2036-Sep-25 | 2464985 | 2036-Oct-18 | 24 |
| 2473765 | 2060-Nov-01 | 2473799 | 2060-Dec-05 | 35 |
| 2476000 | 2066-Dec-15 | 2476006 | 2066-Dec-21 | 7 |
| 2480048 | 2078-Jan-14 | 2480051 | 2078-Jan-17 | 4 |
| 2482443 | 2084-Aug-05 | 2482450 | 2084-Aug-12 | 8 |
| 2496457 | 2122-Dec-19 | 2496488 | 2123-Jan-19 | 32 |
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.
