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In astronomy, one of the most prized pieces of data is to determine the distance to an astronomical body. Prior to the sixteenth century, the distance between planets and the Sun was an educated guess, but an accurate value had not been determined. Without the ability to pace off the distance or use a physical measuring stick, there was no direct way to determine this. How far was the Earth from the Sun? The distance between the Earth and Sun, known as an astronomical unit, was a key piece of missing data. In 1761, one of the first international scientific endeavors was carried out. Ships carrying scientists from numerous countries were dispatched to various observation locations to observe a relatively rare event. The planet Venus was going to pass between the Earth and the Sun, and, from our point of view, would move across the solar disk.
Typically, Venus moves between the Earth and the Sun about every year, but due to the inclination of the orbits of both planets, Venus usually passes just above or below the Sun from the Earth’s point of view. Every now and then, the orbits are lined up in such a way that when Venus passes the Earth due to its faster orbital speed, Venus moves across the Sun’s visible disk. Typically this alignment happens in pairs separated by 8 years, every 100+ years. The exact time period varies. Several past Venus transits are listed below, with some interesting historical notes:
- 1631, 1639—Jeremiah Horrocks makes the first observation of a Venus transit and discovers they happen in pairs separated by 8 years
- 1761, 1769—Large international cooperation
- 1874, 1882—Simon Newcomb makes best-to-date measurements of AU
The same thing can happen for any astronomical object that orbits the Sun on an orbit that is inside the Earth’s orbit, for example, Mercury. There is a Demonstration on Mercury transits that shows how these orbits have to line up just right to observe a transit.
The intent of the international cooperation in 1761 was to observe the transit of Venus from many different vantage points on Earth and measure the tiny shift in timings observed. This parallax effect is similar to what you see when you hold a finger up in front of you against a reference point and observe with one eye at a time. As you blink one eye closed and the other open, your finger appears to shift in position relative to your reference point. The amount of shift is determined by the distance between your eyes, finger, and reference point. The same can be done for Venus by coordinating when it makes contact with the edge of the Sun. By measuring the slight shift in timings of the various observations, parallax could be accurately measured and the distance determined.
In actuality, the needed precision was very difficult to measure because of something known as the “black drop effect.” Hold your thumb and index finger up to a light and bring them very close to each other. Just before they actually touch, it appears as if a drop of oil connects the two. This makes it hard to know the exact moment of contact and makes the timings fuzzy. Venus’s atmosphere combined with other optical effects added a significant source of error to the measurements.
The Venus transit, unlike the Mercury transit, usually gets more attention due to its historical significance, and on June 5 and 6 of this year, Venus will once again move across the face of the Sun. This is the second Venus transit of the most recent pair of transits, the last transit being in 2004.
Many local astronomy clubs or observatories will be hosting public observation sessions to celebrate this historic event. If you don’t get a chance to see this event yourself, you can use Wolfram|Alpha to see how the event will unfold.