Lunar Occultations

Talk given in 2005, accompanied by maps and planet data sheets for projection ].


   A while back I gave a short talk on graze occultations involving the Moon and a star.

    Less frequent, but rather surprisingly common, are lunar occultations of  planets. 2005 sees no less than 11 such events, 7 of Jupiter, 2 of Mars, and 2 of Venus, visible from certain areas of the Earth. As the Moon is only 2160 miles in diameter, the area of Earth that sees the occultation is similarly limited, and a lot smaller near the Poles, or if the Moon is rising or setting. (See map).

    As is so often the case the UK is unlucky. 9 out of the 11 of these occultations are visible only from the southern hemisphere, and the two northern hemisphere events are visible only in Alaska and Eastern Siberia. For those members who enjoy travelling the world for these events, the Siberian opportunity (occulting Mars) is the nearest, on the 12th of December. Pack a good coat and gloves, and while there invest in a fur hat !

    Much rarer and more exotic are mutual planetary occultations, when one planet passes in front of another, as seen from the Earth, not involving the Moon at all. Normally, of course, the nearer planet will pass in front of a more distant one, but Mercury can pass in front of Venus and vice versa, as seen from Earth.

    This is extreme astronomy; an obscure subject and not for the faint hearted. Of the two sources of information I could find, one was entirely in German.

    Mercury is the fastest moving planet and does many, but not the most occultations, and these are hard or impossible to observe due to Mercury’s small size and proximity to the Sun.  Mercury’s angular diameter is 5 to 9 seconds of arc, and it takes 0.24 of a year to complete an orbit. It will be involved in 12 occultations over the next 1000 years, 4 of which will happen within 27 years during the second half of this century. Each lasts only a few minutes.( Details of the planets, with dates and comments, are on separate sheets.)

    Venus. The second part of this century will see not just the 4 Mercury events but on the 22nd November, 2065, Venus will occult Jupiter. Younger members could well be around for this. With these two brightest planets it is potentially as fine an occultation as possible but unfortunately we shall see nothing because it will occur only 8 degrees from the Sun. There is another, in 2023, but that happens only over the Pacific Ocean. With its apparent large diameter and greater range of movement Venus will achieve 19 occultations in the next 1000 years, involving all the other planets. Its angular diameter is 15 to 60 seconds of arc, with 0.62 of a year per orbit.

    There are 4 Venus / Jupiter occultations that have occurred from Roman times, but only one that was ideal. This took place in 2 BC, was 45 degrees away from the Sun, and was visible in the Middle East. Could it have been the Bethlehem Star?                 

    One, in 1210 was, like the future 2065 event, too close to the Sun to be seen.In1570 and 1810 it was only visible in unknown / unpopulated areas of the world. 

    Because of their apparently slow movement, occultations incurring only outer planets are rare. Mars, with a 1.88 year orbit, and angular diameter of 14 to 25 seconds of arc, will occult Jupiter in 2223, 2478, and 2959, but will not occult Saturn until 3332.

   Jupiter will do a spectacular occultation with Saturn, but not until the year 7541, although there will be two 60 second angular diameter near misses before then, in 4523 and 6687. Jupiter will occult Neptune in 3428 and Uranus in 6707. Its orbit takes 12 years, and its angular diameter is 43 to 47 seconds

   Saturn orbits every 29.5 years, has an angular diameter of 17 to 19.5 seconds of arc (minus rings), and will occult Neptune in 6257.

   Uranus orbits every 84 years and has a diameter of 3.7 seconds of arc, while Neptune orbits in165 years and has an angular diameter of only 2.5 seconds. I could find no occultation dates for these. Nevertheless, bearing in mind their angular diameters, their slow progress, and the fact that planets can depart from up to 10 degrees above and below the orbital plane of the solar system, we can work out an approximate frequency for this which is about every 1.3 million years.

    What about a Venus / Jupiter / Saturn triple occultation ? Venus occults Jupiter as Jupiter occults Saturn. From the charts a Jupiter / Saturn occultation occurs only once in rather more than 7,500 years, but about 6 Venus / Jupiter occultations happen within every 1000 years. So the triple can be worked out. I won’t take you through all the mathematics now, but it will be included in a version of this talk I hope to publish in “Eclipse”. Anyway its about 5,000 million years, so we’ve had one or two since the solar system formed - and you can be sure at least one was too close to the Sun to see. ! 

     How about all the planets and their satellites lined up in a perfect mutual occultation ? And throw in a large rectangular dark slab 9 X 4 X 1. Don’t ask me - ask a specialist ! Some sceptics may say all this is trivia, but I call it extreme observing ! And until now I have not once mentioned cloud or rain presenting future challenges. 

   To show what the observer is up against here are three acetates.

   No.1. A map of the best lunar planetary occultation of 2005, with Jupiter, and visible from the largest area of the Earth’s surface. You will notice that this is nearly all ocean. 

   No.2. A list of the lunar / planet occultations for 2005, (11 in all).

   No.3. The German website summary of planet / planet occultations between 1800 and 3100. The first two, 1808 &1818 are long gone.

   Next. 11 / 2065, Venus / Jupiter, will be too close to the sun to see.

              7 / 2067,  Mercury / Neptune, occurs only at the North Pole, which has 24 hour daylight.

              8 / 2079,  Mercury / Mars, visible for two and a half minutes at sunrise in the Middle East and Caspian areas.

             10 / 2088,  Mercury / Jupiter, too close to the sun.

               4 / 2094,  Mercury / Jupiter, too close to the sun.

               8 / 2104,  Venus / Neptune, telescope required.

               9 / 2123,  Venus / Jupiter, visible only over the Pacific Ocean.

               7 / 2126,  Mercury / Mars,  OK.

             12 / 2133,  Mercury / Venus, very close to the sun.

             12 / 2223,  Mars / Jupiter,  OK. (836 years since the last similar occultation).


     Finally, as a sting in the tail, here’s a rhyming puzzle :-

                                            The beginning of eternity                                      

                                            The end of time and space

                                            The start of every end

                                             And the end of every place.

What is it ?

The letter ‘e’.

Thank You.