Planetary transits
The last transit of Mercury over the solar disk occurred on November 11, 2019. The next one will take place on November 13, 2032.
The last transit of Venus over the solar disk occurred on June 5, 2012. The next one will take place on December 11, 2117.
What is a transit?
A transit is the passage of a planet in front of the surface of the Sun. From the Earth, only the transits of the innermost planets can be seen: Mercury (about 13 per century) and Venus (about 13 per millennium).
They occur when the Sun, the inner planet and the Earth are perfectly aligned. The rarity of these phenomena is due to the slight inclination between the orbits of the planets.
Precautions when observing the transit
You should NOT observe the Sun directly, NOR with homemade methods (film negatives, smoky crystals...). You should NOT look through a telescope or binoculars that are pointing directly at the Sun. ALWAYS use adequate protection: eclipse glasses and solar filters for optical devices.
We can choose to observe the image of the Sun projected on a screen. Given the smallness of Mercury's disk, the transit is not perceptible to the naked eye (with eclipse glasses). To obtain a bigger and clearer image we can use a small telescope or binoculars (which can stand the intensity of the Sun's light) to project the image on a white screen. It is advisable to let the device cool down every few minutes of observation in order to avoid damaging it, especially its eyepiece; remember that a transit is a phenomenon that passes slowly over several hours.
Did you know?
Planetary transits have been of capital importance in the history of astronomy. During the 18th and 19th centuries, international expeditions were organized all over the planet to measure the duration of transits from different points on the Earth's surface and, from these measurements, to determine the distance to the Sun.
At present, transits are the main method for detecting extrasolar planets.
Do you want to know more?
Tránsitos. La medida del Sistema Solar y de otros sistemas planetarios
Author/Authors: Pere Planesas Bigas
Date of edition: 2019
Number of pages: 48
Summary of the book:
How was the distance to the Sun determined? How was the anomaly of Mercury's orbit discovered, which would be the first phenomenon explained by Einstein's general theory of relativity? How are most of the orbits of the planets revolving around other stars discovered and characterized? The answer to these three questions is the same: through transits.
The measurement of the great distance from the Earth to the Sun is achieved in the modern era through a combination of Halley's ingenuity, new instruments and the enthusiastic participation of hundreds of professional and amateur astronomers in the transit of Venus.
Download book: «Tránsitos. La medida del Sistema solar y de otros sistemas planetarios».