The earth is the third planet from the sun, our parent star, at a distance which has allowed liquid water on the earth’s surface and life to eventually proliferate. At our distance from the sun, the earth completes one full revolution once every 365.25 days – a period of time which we define as one year. Closer to the sun, planets Mercury and Venus orbit the sun faster, completing one revolution in 88 days and 225 days respectively.

This puts us here on earth in a unique position to be able to observe close-up views of these two planets’ transits in front of the sun from our perspective. A Mercury transit was visible from several places on earth, including Malta, this year on November 11.

Such transits don’t occur frequently, and are only possible when the earth, the sun and the transiting planet, be it Mercury or Venus, are perfectly aligned in their orbits. Since the orbital planes of earth, Mercury and Venus are not perfectly in line, we rarely get to see a transit of either planet. In most cases, Mercury and Venus will pass at their closest to earth just above or below the sun from our perspective, as their orbits are inclined with respect to that of earth. Periodically, however, earth and one of the two planets closer to the sun align perfectly with the sun, and such a transit can be seen from locations on the earth’s surface facing the sun – on the daytime side of the earth at the time of the transit.

For Mercury, such transits can currently occur either in May or in November, since these point in earth’s orbit correspond with the passage of Mercury at inferior conjunction – the point at which Mercury and earth are at their closest. Intervals of six, seven, 13, 20 or 33 years can be present between November transits, while 13, 20 or 33-year intervals will be present between May transits, given the earth’s and Mercury’s orbits. Following this November’s Mercury transit, for example, the next November Mercury transit shall occur on November 13, 2032. Another Mercury transit will then occur on November 7, 2039 (seven-year interval), followed by a May transit on May 7, 2049 – 33 years after the last May transit which occurred on May 9, 2016.

Such transits have been extensively useful for scientific investigations. One such example is the measurement of the dip in brightness of the sun as Mercury or Venus transit in front of it from our perspective. This has made it possible to more accurately estimate what brightness dip can in turn be expected for stars having transiting exoplanets. This has helped in our observations of exoplanets using the transit dip method.

Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also the President of the Astronomical Society of Malta.

Sound bites

• Scientists want NASA to send a flagship mission to Venus: NASA has asked a team of scientists to figure out what they could learn from sending a major mission to Venus and how such a mission might work. Last month, the team became one of 11 groups that NASA announced it would fund to study potential mission designs. Dubbed Venus Flagship, the mission would aim to answer questions about how habitable our evil-twin planet once was. Mission principal investigator Martha Gilmore, a geologist at Wesleyan University in Connecticut, offered an introduction to the project at a meeting on November 8 of the Venus Exploration Analysis Group, which advises NASA.

• First global geological map of Saturn’s Moon Titan released: To create the global map, Rosaly Lopes (Caltech) and her colleagues combined data collected by NASA’s Cassini mission, which flew by the moon more than 120 times. In addition to high-resolution radar mapping that covered almost half of the moon’s surface, the spacecraft also characterised the surface using lower-resolution radar, infrared images and infrared spectra. High-resolution radar captured details as small as one kilometre across. By filling in the full globe, even at lower resolution, the map not only charts the diverse geological formations on Titan, but it also enables planetary scientists to see how those features evolved over time.

For more soundbites listen to Radio Mocha every Saturday at 7.30pm on Radju Malta and the following Monday at 9pm on Radju Malta 2

Did you know?

• One day on Venus lasts longer than its year. Venus has the slowest rotation of any planet, taking about 243 days to complete one whole rotation on its own axis. Its year – the time it takes to make one complete revolution around the sun – is 225 days. Additionally, Venus is the only planet that rotates on its own axis from East to West, rather than from West to East as with all the other planets apart from Uranus – which rotates from North to South.

• Venusian transits are even rarer than Mercurian ones. As seen from the earth, pairs of transits of Venus occur within eight years of each other, but each successive pair is separated by either 105.5 years or 121.5 years. Thus, although the last pair of Venusian transits occurred in 2004 and 2012, the next pair of transits will occur in 2117 and 2125! Interestingly, a Venus transit and Mercury transit can occur at the same time, but exceedingly rarely so; the next double transit will occur in the year 69163!

• An observer on Mars would be able to see a transit of the earth in front of the sun. Although no one has ever ventured to Mars, it is possible for future inhabitants of the red planet to view our own planet transiting the solar disc. The last such transit of the earth as seen from Mars occurred in 1984, and the next will occur in 2084.

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