Conjunctions and Occultations

Conjunctions

A conjunction is the name given to occasions when two or more heavenly bodies come close to one-another in the sky. They are never physically close, of course - conjunctions are simply a "line-of-sight" effect when objects just happen to be in the same area of sky as seen from Earth. Planetary conjunctions are not that uncommon, as all the planets except Pluto orbit the sun in more-or-less the same plane and hence appear in the same small strip of sky as seen from the Earth. An approximate calculation of how often a given pair of planets should come close together in the sky can be made from their orbital periods but this simple picture is complicated by the fact that the Earth is moving as well, which will change the exact time at which the line-of-sight is established and by an amount that will vary according to the particular circumstances of each conjunction. Experimentation with my astronomical program seemed the best answer here, which gave the (averaged) results below for the planets further away from the sun than the Earth:-

Mars-Jupiter:

2yrs 83days

Mars-Saturn:

2yrs 9days

Jupiter-Saturn:

19yrs 83/4months

It isn't possible to give a simple number in the case of Mercury-Venus conjunctions because Mercury's orbit is markedly elliptical and somewhat tilted, which affects its position in the sky very greatly. The more complicated geometry of conjunctions involving Mercury/Venus and Mars, Jupiter & Saturn also makes the derivation of a single repeat interval impossible: the best one can say is that, as the inner planets stay fairly close to the sun in the sky and the sun makes a round trip in one year, there is likely to be a conjunction of each inner planet with each outer planet about once per year.

The main interest in conjunctions is how close the planets get and how many are involved in a given conjunction. The conjunction between Mercury and Venus described in the "Mercury pictures" page was a very close one - only 4¹/2 minutes of arc, in fact - or less than one-sixth of the diameter of the full moon (which is about ¹/2 degree, or 30 minutes of arc). Anything within a couple of degrees will be notable but there's really no definition of how close an approach must be to be called a conjunction. Clearly the most common conjunction will involve just two planets but triples do also occur reasonably often - more than this is unusual though. Incidentally, while a coming-together of two planets is correctly called a conjunction, an event involving three is not called a triple conjunction! This term is reserved for occasions when two planets come close three times in a short period (yes, it can happen but no, I'm not going to elaborate). The correct term for "multiple conjunctions" is a grouping or massing. I shall continue to use the "wrong" term below though!

Upcoming events

Remarkably, all possible conjunctions (with the exception of Jupiter-Saturn) happened in the space of just 13months from February 2008 to March 2009.

The next triple is Mercury, Venus & Jupiter on the morning of 12th May 2011: this is in fact also the next quartet as Mars is nearby. This means that for the next triple after that [Mercury, Venus & Mars] you need only wait until the 21st. These "massings" are rather close to the horizon though (only 6deg above at sunrise). A "full set" of all five close to each other will not occur until 8th September 2040! Worth waiting for, as it will also include a 2-day old crescent Moon, but very low down in the evening sky as seen from the UK.

Conjunctions between the major planets are not, of course, the only type of conjunction. Planets can come into conjunction with stars; the moon can come into conjunction with planets and stars, and the asteroids [small bodies orbiting between Mars and Jupiter] can have conjunctions of their own. Conjunctions with stars don't tend to be particularly dramatic events unless the stars are quite bright and of course lunar conjunctions can be overwhelmed by the great brightness of the moon: conjunctions with the crescent moon can be quite spectacular though (see below for a conjunction with Venus). Lunar conjunctions can be useful for finding planets further out than Saturn, however, as the close proximity makes the identification of the correct "dot" much easier. See my gallery of pictures of Uranus for an example of this.

Conjunctions involving asteroids and the outer planets are relatively uncommon, as their orbital periods are quite long (3.6 to 4.6yrs for the largest ones) and their orbits tend to be inclined to the plane of the solar system so they often travel outside the band of sky traversed by the planets: there was one between Vesta and Jupiter in August 2007 though (see my Asteroids page for further information). Because of their similar orbital periods and inclined orbits, conjunctions between two (large) asteroids are among the rarest of planetary phenomena: just three events in the 21st Century! Conjunctions with Mercury and Venus happen about annually (as with "major" planets) but, as these will be near the sun, in practice they are very difficult to observe.

Occultations

The most extreme conjunction is when two objects come so close that their discs overlap: this is called an occultation. Strictly, it is only an occultation if the "foreground" object is (apparently) bigger then the "background" object: if the reverse is true the event is called a transit. One could thus say that an annular eclipse is a transit of the moon across the sun and a total eclipse is an occultation of the sun by the moon! Occultations of one planet by another are exceedingly rare [last one was in 1818, next is not until 2065!!] but occultations by the moon happen more frequently as the disc of moon is (clearly!) very much bigger than that of any planet. The downside is that, like solar eclipses, they are not visible from all of the Earth's surface. In the case of the sun, apart from eclipses the only events actually visible are transits by the inner planets Mercury and Venus: see my pages on eclipses and transits for more information on these types of events.

Just as the planets in the solar system can be invoved in occultations and transits, so can the satellites of the larger planets, acting as sort of "mini solar systems". In practice, only those involving the Galilean moons of Jupiter are easily observable but they offer all types of phenomena: eclipses; transits across the disc of Jupiter; occultations behind it, and occultations & transits of one moon by another. See my section on Jupiter for more information.

Upcoming occultations of planets by the moon, visible from the UK
[calculated for north Suffolk, taking into account sunrise & sunset times and elevation above the horizon]

Pictures of Conjunctions

So onto some pictures of actual events, conjunctions first then an occultation at the end - worth waiting for though! The first is of the Mercury-Venus conjunction of 27th June 2005 mentioned on the "Mercury" photos page, taken using an 840mm lens on a 35mm film camera:-

Here we see Mercury and Venus in the rapidly fading twilight of a rosy sunset. And no, that is not a cactus at bottom right - I know we had a dry June but that's just ridiculous! The grey disc on the right represents the full moon at the same scale: the separation was just less than 5 arc-minutes (1/12 degree).

		The Venus-Mercury conjunction was actually a triple conjunction a few days earlier but, inevitably, it was cloudy here around that time so here's a picture taken from the Internet, at the same scale as those below, showing Saturn, Venus & Mercury on 25th June. The maximum separation (Saturn-Mercury) is 12/3 degrees.
And here's lots more conjunctions, taken with a digital camera: note that the scale is about one third that of the Mercury-Venus picture above! The grey discs again show the size of a full moon for comparison.
Venus and Jupiter, captured by Sandra in the evening dusk at Laxfield on 2nd September 2005. The separation is 11/2 degrees.		Mars and Saturn on 22nd June 2006, separated by 21/3 degrees. They were much closer a few days earlier (just over half a degree) but, you've guessed it, it was cloudy for several days here!!
In August 2006 there was a double conjunction at dawn involving Venus and Mercury & Saturn. Although the three did line up (twice, in fact), one couldn't really call these triple conjunctions as the separations were all over 4degrees. Apart from which, it was cloudy (again) so I didn't see them anyway! The pictures above show (at the same scale as before) the minimum approach of Mercury to Venus on the 10th (21/4 deg.) and the very close approach of Venus to Saturn on the 27th (just 13 arc-minutes). Both pictures were taken at 5am (!), showing how much Venus had moved between the events - compare against the same tree on the right.
I tried to see the close grouping of Jupiter, Mercury and Mars on 10th/11th December 2006 but, although I spotted Jupiter and Mercury in the binoculars on the 10th, I didn't notice that, because it was so cold that morning, the lens of the camera had slightly frosted up! Result - no usable pictures. It was then totally cloudy and rainy on the 11th so I tried again on the 12th, to be rewarded with a perfect dawn. We thus have, from the left, Mercury Jupiter and Mars: separations are 2 degrees and just under 1 degree respectively. Would have been 3/4 degree the morning before but hey, at least I saw it!

	After being close together in the morning sky in August 2006, by July 2007 Saturn had passed through opposition and was heading back towards the sun again and Venus had also "swapped sides", as it were, being now a brilliant evening star. The two were thus able to come back into conjunction but at sunset this time. Despite a really terrible period of bad weather I was fortunate enough to be able to catch them at minimum separation (3/4 degree) late in the evening of the 1st.
The period February 2008 to March 2009 was quite a special one because all possible conjunctions (with the exception of Jupiter-Saturn) happened during this time. Here's the first event, Venus above Jupiter just before dawn on 1st February - the separation is 2/3 degree. These two planets give us the brightest pairings so the view was quite spectacular: until the cloud closed in, that is!
	This game is called "spot the planet". The planet in question is a very bright Venus, so it can't be a problem, can it? Well yes it can, as a rising sun and even the thinnest cloud at this elevation above the horizon (just 31/2deg) conspire to blot most things out. When you give up, hover the mouse pointer over the picture to see a heavily contrast-enhanced version of the image, just to prove it really was there! I've included this image, of the Venus-Mercury conjunction at the end of February 2008, to show how difficult it can be to observe near to the horizon and close to the sun: and no, I haven't been able to find Mercury either! It should have been above and to the right of Venus but at fourty-five times less bright than Venus at the time I don't think I had much chance of capturing it under these conditions.
I had been watching Mars approach ever closer to Saturn during the early summer of 2008 and here they are at their closest, late in the evening of 10th July - the separation is 2/3 degree, with Saturn above Mars. This is the closest conjunction between these two planets until 2022.
Having met in the morning skies in February 2008, Venus and Jupiter came together again in the evening during November and December. This was made even more spectacular by the intervention of the moon, an event dubbed the "Great Conjunction". Read all about it by clicking here.
	After its encounter with Jupiter and the moon early in December, Venus carried on to a conjunction with Neptune at the end of the month. The minimum separation, here on the 27th, was 11/3 degree: Neptune is the middle one of the line of three to top right. The brightest star (bottom left) is Nashira, in Capricorn, but none of the others is distinctive enough to have a name (which goes to show how faint Neptune was! Magnitude 8, in fact). [Note that this image is not at the same scale as the others on this page]. I wouldn't normally try to capture conjunctions with Neptune (or indeed Uranus) as they are not particularly photogenic but thought I would on this occasion as, just after the coming-together of the two brightest planets, here we have the conjunction of the brightest and the dimmest. Using Venus as my guide, I was actually able to capture Neptune on four out of five consecutive nights so have included a fuller description in the "Outer Planets" page - click here to read about it.
Jupiter also had a second conjunction around the turn of the year, with Mercury, always a difficult target to spot. There was just enough cloud on the horizon to prevent me seeing Mercury on the day of closest approach (31st December) and the next two days were competely cloudy. Mercury moves across the sky very quickly though so by 2nd January it had risen quite a lot higher in the sky, thus making it easier to see, but further from Jupiter - the separation by this time (here shown to correct scale) was 21/2 degrees, with Mercury to the left of Jupiter.
	While checking star maps for the track of the ISS in mid-July 2009, I noticed that Jupiter and Neptune were very close in the sky, which gave me another opportunity to capture the elusive outermost planet. This image [actually a "stack" of four] was taken just before midnight on 12th July when the two were just past the point of closest approach, but even so the separation was only a fraction over 1/2 degree. Jupiter is (obviously!) to the bottom of the frame, with its satellites Ganymede, Europa and Callisto also visible, and Neptune is to top left. The only other object visible is the 5th magnitude star, Mu Capricorni. As with the Venus-Neptune conjunction above, this image is not at the same scale as the others on this page - the increased magnification shows Neptune's distinctive light blue colour quite well. The image also again shows how dim Neptune is. Although seeming bright here, the star would be only just visible to the naked eye yet Neptune is 13 times fainter still. Just to prove it was Neptune, I took some further images a couple of days later - click here to see the results.

The term "conjunction" usually refers to planets, but of course the moon can be involved too. Here are some dramatic examples:-
	On the left we have a lovely conjunction of Venus and a very thin crescent moon on the evening of 20th January 2007. The "centre-to-centre" separation is about 12/3degrees, with the moon barely 40hrs old. If you look carefully you can just see the full disc of the moon faintly illuminated by "earthshine": light reflected from the earth back to the moon. On the right, the moon is even "younger" - 32hrs - and the earthshine is much more obvious as the image was taken later into the twilight, on 6th May 2008. The planet on this occasion is Mercury, with the separation being a fraction over 2deg.
	And a "triple conjunction", of sorts! Here we have Mercury below the moon on 26th April 2009 with the Pleiades above it - taken later in the evening than both those above, earthshine is very noticeable. The moon was just 29hrs old, with the separation from Mercury being 2 degrees and from the Pleiades 11/3 degrees.

All the conjunctions above have been either at sunrise or sunset. To some extent this is just chance, because except for Mercury (and to a lesser extent Venus) all the planets can also be involved in conjunctions during the hours of darkness - the images are not so photogenic though, due to the lack of foreground! Here we have the moon again, in very close conjunction with Saturn at 11pm on 2nd February 2007. The centre-to-centre separation is less than 1/2degree: "edge-to-edge" it is only 12 arc minutes. The moon is just past full - about 15hrs in fact. On the left the two are at the same scale as the images above - note that the grey discs really are the size of the full moon, as claimed! On the right is a larger-scale image showing how the conjunction looked through binoculars - the great brightness of the moon meant it was impossible to photograph it and Saturn at the same exposure so this is a montage of two images taken separately. The moon actually occulted Saturn the next month - see below.

Pictures of Occultations

Saturn

Occultations don't happen very frequently so I was keen to capture that of Saturn by the Moon on 2nd March 2007. The weather had been very variable in the week previous but luckily cleared just on the evening of the occulation. The downside was that the event occured at 2:35am! Ah well, are we dedicated or not? Despite the best planning, however, things can go wrong and indeed they did. The worst near-disaster was when the web-cam slid partly out of the eyepiece tube thus ruining focus and resulting in fuzzy images. At first I put this down to bad atmospheric conditions and didn't deduce the real answer until after the "inward" half of the occultation was over. The images were, however, good enough for me to be able to reconstruct what I would have seen had I noticed the problem sooner.

Fortunately however, occultations have two parts and so I was ready for the "outward" half. This was also missed as well though, as I was initially looking at the wrong section of the moon's limb! I realised just in the nick of time though, so was able to get a pretty good series of Saturn emerging from behind the moon. Exposure was something of a problem, as the moon is so much brighter than the planet, so I concentrated on getting things right for Saturn and took a few shots of just the moon later so I could combine the two digitally to give an accurate "visual" representation.

		On the left we have Saturn "flying" above the moon's cratered surface some minutes after having emerged from behind its limb. The dark area to the top is Mare Nectaris, with bright spots on each side of the crater Stevinus particularly prominent at right-centre. The animations below show Saturn disappearing and then re-appearing after the occultation: each sequence represents about 5mins in real-time. Note that in reality the moon was actually moving in front of Saturn, not Saturn behind the moon: showing it the latter way made it easier to align the frames of the animation though!
		The prominent crater behind which Saturn is "disappearing" on the left is called Cabeus: note the sunlight glinting on the central peak. The feature to upper middle is a circular mountain range called Drygalski. On the right, the dark patches are part of Mare Australe, with the dark crater Oken to far right.

Mars

The moon's meeting with Mars in the early morning of Christmas Eve 2007 wasn't actually an occultation (i.e. the moon did not cover any part of the disc of Mars) but was so close - just under 2minutes of arc - it was categorised as a "near miss" instead of a simple conjunction. The weather was most unhelpful here unfortunately, with complete cloud cover for a day or so beforehand, so I was not particularly hopeful. I got up at 3am anyway (yawn), and did manage to catch a few glimpses of the moon through scudding broken cloud so thought it might be worth setting up the equipment. The main telescope plus webcam didn't prove to be usable though, as firstly the highly variable cloud caused the exposure to be all over the place and secondly my laptop (used to capture the images) chose this moment to shut down and refuse to turn on again!! Mr Murphy and his Law were having an early Christmas! I thus transferred to the digital camera at full magnification (800mm equivalent) which at least meant I could choose my moment to take a shot - the exposure was still guesswork though!

All this meant that very few images were usable - just three, in fact. Not particularly impressive, but better than nothing! By chance, these were at about the time of closest approach so at least I got something to show here. I have made them into an animation and a composite picture (in black&white for a clearer view). The first image was taken at 3:44am, then 3:50 and 3:55 - closest approach was 3:46am. As with the animations of Saturn above, it was really the Moon moving, not Mars, but the animation looks better this way round.

So, just another 15yrs to wait for the next event then!