Transits of Venus

Seventeenth century astronomers had obtained a lower limit for the Earth-Sun distance. A new method used during the next two centuries was the passing of Venus across the Sun's disk. This method has since been replaced by more accurate techniques, but it has an important place in the history of astronomy and as the first extensive international collaborative research project.

When it goes around the Sun inside the Earth orbit, Venus sometimes passes through the Earth-Sun line and is then seen on the disk of the Sun as a small dark spot. Such transits are quite rare, but then they happen in pairs separated by 8 years as given below:

6 December 1631 6 June 1761 9 December 1874 8 June 2004 4 December 1639 3 June 1769 6 December 1882 6 June 2012

1 Support for the large distance also came from across the English Channel. James Flamsteed (1646-1719) used a method suggested by Tycho Brahe. He followed the movements of Mars in the starry sky for several hours. Its apparent motion reflects the orbital motions of both Mars and the Earth. The daily rotation of the Earth also causes a shift which is smaller the more distant Mars is. Thus Flamsteed also concluded that the solar distance must be "at least 21,000 Earth radii."

Fig. 9.2 The transit method. Venus crosses the Sun along different lines depending on the geographic latitude of the observer on the Earth. As the ratio of the Earth-Sun and Venus-Sun distances is 7:5, the apparent trajectories can differ no more than 5 Earth radii, meaning about 44 s of arc on the Solar disk. We greatly exaggerate this difference in the drawing (remember that the diameter of the Sun is actually half a degree, 40 times the maximum difference). In essence, the Sun is used as a background to measure accurately the parallax angle of Venus, after which the distance of the Sun is obtained from the 7:5 ratio

Fig. 9.2 The transit method. Venus crosses the Sun along different lines depending on the geographic latitude of the observer on the Earth. As the ratio of the Earth-Sun and Venus-Sun distances is 7:5, the apparent trajectories can differ no more than 5 Earth radii, meaning about 44 s of arc on the Solar disk. We greatly exaggerate this difference in the drawing (remember that the diameter of the Sun is actually half a degree, 40 times the maximum difference). In essence, the Sun is used as a background to measure accurately the parallax angle of Venus, after which the distance of the Sun is obtained from the 7:5 ratio

Transits are seen either in June or December, when the Earth passes those points in its orbit where the slightly inclined orbit plane of Venus cuts the plane of the Earth's orbit. Edmond Halley realized the possibility of measuring the Earth-Sun distance with transits in 1716 when he observed a similar event for Mercury. However, he did not live to see its first application to Venus in 1761. The idea was that observers stationed in widely separated geographic latitudes would observe the transit and measure accurately the interval of time it takes Venus to cross the Sun's disk. Observers at southern latitudes will see Venus cross the Sun closer to its northern pole than those observing at northern latitudes. The time intervals give the precise positions of the trajectories of Venus on the solar disk. Combining that knowledge with the known geographic latitudes of the observers and the known ratio of the orbit sizes of the Earth and Venus leads to the distance of the Sun (Fig. 9.2).

The measurement procedure is surprisingly simple, requiring only a telescope and a good clock. However, the observers were not happy to find that the timing of the moment when Venus within the solar disk touches the edge of the Sun could not be made as accurately as was hoped, since the image of the touching point becomes fuzzy. Related optical phenomena gave the first indication that Venus has an atmosphere (see Fig. 9.3. and Chap. 31). Since the accuracy of timing is critical for the method, the results of the 1761 and 1769 transits did not quite reach the level of precision hoped.

The observations of the second transit were carefully planned. There were 77 observing stations all around the Earth and 151 observers. It took decades to analyze and combine all the observations. The end result was that the distance to the Sun is 24,200 (±250) Earth radii. More modern determinations with different methods give the more accurate result 23,494 Earth radii. One needs the size of the Earth in meters to complete the calculation of the Earth-Sun distance.

Fig. 9.3 (a) The 2004 June transit of Venus, 8 June, 11 UT (Credit: USNO). (b) Venus on the limb of the Sun. The brightening of the edge of Venus' disk against the black sky is caused by refraction in the thick Venus atmosphere (credit: Dutch Open Telescope at La Palma, Utrecht University)
The Art Of Astrology

The Art Of Astrology

Get All The Support And Guidance You Need To Be A Success With Astrology. This Book Is One Of The Most Valuable Resources In The World When It Comes To A Look at Principles and Practices.

Get My Free Ebook


Post a comment