A transit is a cool astronomical phenomenon that can be likened to a mini-eclipse. As you can imagine, it is similar to the everyday use of the word; as you, the reader, travel from home to school or office every day, during transit, a planet travels from one end of an astronomical object to the other.

Now, the reason that I have chosen this topic is, of course, because of the transit of Mercury that occurred on the 11th of November in 2019. For two hours between 11 am to 1 pm, observers (including myself!) were able to see Mercury slowly drifting across the vast face of our star. Here is a picture that a fellow volunteer took through a telescope with a solar filter:

Credit: Ken McAdams

The huge white circle is the Sun, and the tiny black dot at about 1 o’clock is the planet Mercury. It may look like an offending grain of dust, but it is actually the smallest planet in the solar system, only about 25% bigger than our own moon.  

The whole process was an almost spiritual experience, and I was reminded once again why I love this science so much. It was mesmerizing to watch what looked like an imperfection in the telescope creep across the Sun bit by bit until it had completed the almost five-hour journey. 

But transits have plenty of cool things going on behind the scenes. There are some fascinating orbital dynamics that go into their formation and some even cooler things that can be discovered by studying transits. For example, did you know that we can use transits to estimate the size of the solar system?

So, without further ado, let us dive into the complex and fascinating world of a planetary transit.


Transits, as I said earlier, are like mini-eclipses. Incidentally, because of this feature, solar transits can only happen with two astronomical bodies total: Venus and Mercury. Both of which, of course, are the only two astronomical bodies between us and the Sun. 

The moon is so close to the Earth that it can completely block out the Sun. You can liken this to eyelids; even though they are minuscule compared to your field of vision, they are so close that they can completely cover up what you are looking at. Venus and Mercury are like two pencils you are holding out farther forward; they are blocking your vision somewhat, but not as much as something much closer. 

There are several interesting scientific conclusions that can be drawn from observing an eclipse. The first realization about the potential of transits was made by the famed Edmund Halley (yes, after whom the comet is named). After observing a transit of Mercury, he realized that the parallax effect could be used to determine how far away Mercury—and, as a result, the Sun—actually was. This would be momentous, as one of the biggest problems in astronomy for that era was the lack of understanding of the true scale of the solar system.

Halley, however, would not be the person to make the calculation. 25 years after Halley’s death, in 1769, scientists observing the Venus transit were able to achieve what he could not; using a team of observes positioned on the opposite ends of the Earth, astronomers were able to construct imaginary right triangles and were able to use basic high-school trigonometry to find the distance. How cool is that?

You may be thinking, “But do we really need that information? It’s just distance and scale!” However, knowing the distance to the sun is, in fact, quite valuable to astronomers. One of the lesser-known units of distance is an “astronomical unit”, or AU, and is defined as the distance from the Earth to the Sun—or about 149,597,870,700 meters. This is used by astronomers typically when discussing exoplanets; if a planet is within 1 AU of its star, who knows? It may be habitable in the future! 

Another really cool discovery that can be made when viewing transits ties back to my last blog post about light. In it, we discussed the various aspects of an object that can be discovered through spectroscopy—or the analysis of the light that an object gives off. As expected, if one is to analyze light, there needs to be light in the first place!

This was another problem that astronomers faced. As Mercury and Venus were not on the other side of the Earth, they did not reflect any light at night, and astronomers were not able to do a spectroscopic analysis for either planet. As a result, both planets were shrouded in an almost mystical fog as several scientists postulated what the planets might be like.

But, of course, the solution to this problem also lies in transits! As both orbital planes of the planets are aligned during a transit—not only just the position to the Sun but also planar—the sun shines through the planet, enabling a spectroscopic analysis of the planet.

This process was primarily used for Venus, as Mercury’s atmosphere had been burned up by its proximity to the Sun. By using preliminary spectroscopic analysis, scientists were able to detect that Venus’s atmosphere was made of carbon and sulfur dioxide. This was later confirmed when probes were sent to Venus in the late 20th century. Of course, another discovery was made; Venus is completely inhabitable.

Now, the Sun is not the only vehicle that can facilitate transits. While the only planetary transits that are significantly visible from our perspective will, indeed, be of the sun, there are other transits that occur throughout the solar system. 

As I mentioned earlier, a transit is defined as an object passing from one end of any astronomical object to the other end. As a result, there are many more common transits that occur further down in the solar system.

Here is one of the most famous pictures of Jupiter:

Credit: Cassini | NASA

This impressive image was taken by the Cassini spacecraft as it flew past Jupiter on its way to Saturn. While it may not seem so, this is also an image of a transit; it is a transit of the Jovian moon Io. 

Other, less conventional transits have also been observed. Spacecraft orbiting the Earth at a distance have seen transits of the moon across the Earth. Rovers on Mars have seen Martian transits as the Earth, Mercury, and Venus have all passed in front of the Sun.

And believe it or not, the modern method of discovering exoplanets is also dependent on transits! By monitoring a star and its luminosity, astronomers are able to understand when a planet passes in front of the star—or transits the star—by noting a slight dip in the brightness of the star. 

Transits are everywhere, from the colloquial definition of travel to and from a place to the astronomical definition of the same travel but across another astronomical object, they are everpresent in our activities. They are gorgeous to view from Earth, as well as exceptionally rare; Venus transits the sun once about every 240 years, and Mercurial transits do not repeat in the same place for 30 to 50 years. But even ignoring all the scientific information that can be gained from these magnificent events, often the most peaceful thing to do is just sit back and watch a planet slowly cross the grand profile of the star that we call ours.

Clear skies!