The hidden…

In a recent tweet, the Sky at Night Magazine mentioned that a letter had been received asking “What is all the fuss about occultations”. These events, when one object in the sky moves in front of another and obscures it may seem pretty routine and ordinary, but they are symptoms of something many people may not give much thought to. Although the stars in the sky look as if they are silent and enduring pinpricks of eternal light, they are constantly in motion. The more distant objects move much slower in the sky as seen from Earth, though their “proper motions” can still be measured by high powered telescopes over time. Closer objects like asteroids, the plants and the Moon, however, are much faster movers and as a result they will often find themselves passing over bright stars and even each other. The three major categories of events this can cause are eclipses, occultations and transits.

Eclipses fall into two categories. Solar Eclipse like events, where an object passes in front of and obscures another of similar apparent size (the size it appears to be to us viewing on Earth rather than its real size). Eclipsing binaries, where one star orbitting another is able to block out the light of its companion as seen from Earth, are one system that does this. The other type of eclipse is Lunar Eclipse like – when a satellite of a planet passes into the shadow of that planet. These are pretty difficult to spot without a spacecraft to simulate things as the inner planets, whose darksides we see easily, have no satellites and the outer planets have very little shadow available for us to see.

Lunar Eclipse

Lunar Eclipse

Transits occur when an object passes in front of another with much larger apparent size. Two inner planets – Mercury and Venus – are capable of transiting the Sun as seen from Earth. Venus last did so on the 8th of June 2004, when the Eddington Society first came into being. The next one will be on June 6th 2012, though only visible in its dying stages during sunrise on that day from the UK. Then there will be a 243 year break before the next pair of transits. Transits of Mercury across the Sun are far more frequent, with roughly thirteen recorded in a century. The next two transits are on the 9th of May 2016 and November the 11th 2019, followed by a long absence. The previous three transits were seen on 15th of November 1999, 7th of May 2003 and 8th of November 2006. However, there are other transits that can be made in the solar system. These include transits of planetary discs by satellites and transits of planets by other planets. The first set are extremely common – the Galilean moons of Jupiter often drift over the gas giant’s face, casting a shadow that can be seen from Earth, this year with the rings close to edge on, Saturn’s in the right position for a similar thing to happen too – perhaps even four at once. The second set are less so, the last transit of one planet over the face of another, and in fact the next transit too, was Venus passing over Jupiter. The last one happened on the 3rd of January 1818 and the next one will occur on the 22nd of November 2065. Transits of the Sun have been seen by probes on other planets. Below is an animation of Phobos, a satellite of Mars, passing between Opportunity the Mars Rover’s camera and the Sun. Deimos has also been captured doing this, though it is a much smaller satellite as seen from the ground. Like in the case of eclipsing binaries, where one star passes in front of the other as seen from Earth, exoplanets in orbit of a distant star can transit that star as seen from here. This can be measured by noting the extremely slight dimming of the star when the planet passes between it and the observer. This dimming is so slight it requires either a very large planet or a very powerful telescope to resolve it. One such mission is the Kepler mission, which today threw off its dust cover, having been guided into the correct orbit.

Phobos transits the Sun

Phobos transits the Sun

The final category are actual occultations, where an object of much larger angular size passes between the observer and an object of smaller size, hiding it completely. As with transits, mutual occultations of planets by other planets are incredibly rare. Planets, the Moon and even asteroids do however occult stars as they move about in the night sky. The Moon is even known to occult planets. It will be a few years before the next time a planet occults a bright star, however the Moon will occult a planet, as seen from various locations on the Earth’s surface, several times a year. Asteroids will occult dim stars several times a day. When two objects of familiar shapes come together, an awesome display of angular sizes and speeds in the sky can be seen. Scientifically, occultations are used to measure atmospheric composition as the different gases in the atmospheres will absorb starlight according to their own spectra. Seeing where and how the starlight dims as it approaches the surface gives information on the gas between the observer and star. Radio astronomers have also used occultations to pin down the locations of radio sources. As their wavelengths give low spatial resolution, the location at which say the Moon blocks out the radio source can be used as a better determination of location than any form of radio image of the sky. Below, an occultation of Saturn by the Moon has been videoed and placed on youtube for all to enjoy. The Society for Popular Astronomy‘s Sky Diary gives times for future major occultations. This website gives the harder to spot occultations by asteroids of dim stars list. Another source of occultations that has become more important as time goes on are occultations of various objects by satellites. Most recently, Saturn was occulted during a pass of the ISS over parts of the UK.


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