Accessible Astronomy: The Transit of Mercury

Get ready to witness a rare spectacle that won’t be seen for another 13 years as Mercury transits the Sun on 11th November 2019. But what is a transit, what is its scientific importance and how can you see it? Affelia Wibisono, PhD student, tells us more.

What is a transit?

A transit occurs when an astronomical object passes in front of another. The transiting body obscures a small portion of the seemingly much larger and distant one. From the Earth, we can only observe two planets transit the Sun – Mercury and Venus – as they are the only ones that can pass in between us and our star. These transits are rare. Typically, you’ll expect Mercury to transit the Sun 13 times in a century even though Mercury completes 4 laps around the Sun for every revolution that the Earth makes.  That’s because the two planets orbit the Sun in similar, but not identical, planes. Mercury’s orbital plane is tilted with respect to the Earth’s, which means that most of the time, Mercury’s orbit lies above or below the Earth’s orbital plane. From our point of view on the Earth’s surface Mercury will appear to be above or below the Sun. Transits can only happen when all three objects are in a straight line, in other words, when Mercury crosses the Earth’s orbital plane. Transits of Venus are even more rare because Venus takes much longer to orbit the Sun than Mercury. In fact, the next transit of Venus will be in 2117 – 105 years after the last!

The orbital planes of Mercury and the Earth. A transit of Mercury can only happen when the Sun, Mercury and the Earth are in a straight line. Image credit: ESO.

What can transits tell us?

In the late 1700s, transits of Venus were used to calculate the distance between the Earth and the Sun by using parallax. An object appears to be in different positions when seen from two different places. Point your index finger upwards at arm’s length and look at it with your right eye open and your left eye closed. Now swap eyes. You’ll notice that your finger seems to jump from one place to another compared to the background. In the case of the transits, the two eyes are two observers at opposite sides of the Earth. They will see the transit start and end at different times with Venus appearing to take different paths. If the distance between the two observers are known, those measurements can be used along with trigonometry to calculate the Earth-Sun distance. Astronomers already knew the relative distances between the Sun and each of the planets, so this new information was key to determine the size of the Solar System.


The TESS space telescope uses the transit method to discover nearby exoplanets. Image credit: NASA



In modern times transits have been crucial in discovering exoplanets – planets outside of our Solar System. Out of the 4000 exoplanets discovered so far, more than 75% of those were detected using this method. Space telescopes such as NASA’s TESS can detect the minute dips in a star’s brightness as an object moves across it. If those dips occur regularly and repeatedly then it’s most likely caused by an exoplanet orbiting the star. We can work out the distance between the exoplanet and their star by knowing the length of time between consecutive transits and the exoplanet’s size can be found by measuring how much starlight it blocks. The European Space Agency’s UCL-led  ARIEL mission will determine the composition of the exoplanet’s atmosphere (if it has one) by analysing which colours from the star’s light are missing after it has passed through the atmosphere.

What will it look like and how can I see it?

Mercury will appear as a small black dot against the bright Sun. Our eyes will not be able to pick up the planet’s small silhouette so a telescope or a pair of binoculars are needed to magnify it. Just make sure to use the appropriate solar filter to protect your equipment, and most importantly, your eyesight! The edge of Mercury’s dark outline is sharp due to its lack of atmosphere. Venus on the other hand has a fuzzy edge because of its thick atmosphere.

The exact time of the transit depends on your location. The show in the UK will start just after 12:30 in the afternoon and will end at sunset at around 16:15. It won’t be visible to everyone in the world as it will unfortunately happen during the night for those in Asia and Australia.

If you don’t have the necessary equipment you could see if your local astronomical society is planning to have a public observing session or you can catch it live online.

The 2016 transit of Mercury as seen from Minnesota, USA. Mercury is the small black dot in the bottom half of the Sun. The blemishes in the top half are sunspots. Image credit: E. Matthews.

Transit watching is a centuries-old activity and many astronomers in the past have been left frustrated by bad weather preventing them to observe the event. Let’s hope for clear skies so that we don’t have to wait another decade to marvel at the dynamics of the Solar System.

Featured image credit: JAXA/NASA/PPARC

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