Chapter 1 · 1619
A map of the planets with no scale bar
Kepler's third law says a planet's orbital period squared equals its distance cubed (T² = a³, in years and Earth-orbit units). Periods are easy to time, so astronomers knew the shape of the solar system very well.
What nobody knew was the size of the unit itself: the distance from Earth to the Sun, the astronomical unit. Every distance on the map was "so many AU", and the AU was a guess.
In 1672 Cassini and Richer measured Mars from Paris and Cayenne and got roughly 140 million km, but the angle they needed was smaller than their errors. Edmond Halley believed a transit of Venus could do much better.
Chapter 2 · 1716
Two observers, two paths across the Sun
Venus passes between Earth and the Sun about twice a century. Seen against the Sun, it is a black dot that takes about six hours to cross.
Observers far apart on Earth look at Venus along slightly different lines, so each sees it cross a slightly different chord of the solar disk. A chord closer to the centre is longer and takes longer to cross. Halley's insight: you don't have to measure the tiny angle between the chords. You only need a clock.
"…by this means the Sun's parallax may be discovered within its five hundredth part."Edmond Halley, Philosophical Transactions, 1716
A · Vardø0h 00m 00swaiting
B · Tahiti0h 00m 00swaiting
Each timer runs from 2nd to 3rd contact, while Venus is fully on the Sun. Clock: —
The real shift between the two chords is about 30 arcseconds, the width of a coin seen from 150 m. The slider exaggerates it so you can see the geometry; the maths in Chapter 5 uses the true size.
Chapter 3 · Plan the expedition
Choose two stations
Halley's rules: the stations should be far apart north to south, and the Sun must be up for the whole transit, at least between 2nd and 3rd contact. You are looking at the Earth as the Sun sees it at mid-transit.
Your next click on a station label or dot sets
North–south baseline, as seen from the Sun
0 km
Chapter 4 · At the telescope
Time the inner contacts
2nd contact: Venus has just moved fully onto the Sun and detaches from the edge. 3rd contact: it first touches the far edge. Their difference is the duration Halley needs.
Watch for the black drop: near the edge, Venus seems to stay glued to the limb by a dark thread. Many 18th-century observers disagreed by close to a minute because of it.
Keyboard: 2 and 3 mark contacts, space starts and pauses.
Chapter 6 · What really happened
Two centuries to pin down one number
Halley died in 1742 and never saw a transit. Hundreds of observers went out for him in 1761 and 1769, to the Arctic, Siberia, Hudson Bay, India and the South Pacific.
- 1639Jeremiah Horrocks and William Crabtree make the first recorded observation of a transit of Venus.
- 1677Halley times a transit of Mercury from St Helena and sees that contacts can be timed to a few seconds.
- 1716Halley publishes the duration method and urges future astronomers to use the transits of 1761 and 1769.
- 1761More than 100 observers. Mikhail Lomonosov sees a bright ring around Venus and concludes it has an atmosphere. Guillaume Le Gentil watches from a rolling ship and cannot time anything.
- 1769James Cook and Charles Green observe from Point Venus, Tahiti; Maximilian Hell from Vardø under the midnight sun. Le Gentil, after waiting 8 years in India, is clouded out at Pondicherry.
- 1824Johann Encke combines 1761 and 1769: solar parallax 8.58″, about 153 million km. The black drop left it 2–3% too large.
- 1882Photographic expeditions for the 1874 and 1882 transits. Simon Newcomb's analysis gives about 149.6 million km.
- 1961Radar echoes from Venus measure the distance directly, to within about a kilometre.
- 2012The IAU fixes the astronomical unit at exactly 149,597,870,700 m. The last transit of the century is observed worldwide.
- You—
Next transits of Venus: 10–11 December 2117 and 8 December 2125.