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The 2016 Transit of Mercury And how to observe it Dr Adrian Jannetta MIMA CMath FRAS The 2016 Transit of Mercury 1 / 27 Adrian Jannetta T HE P LANETS 2 / 27 O RBITS OF THE INNER PLANETS Mercury and Venus can pass between the Earth and Sun (inferior conjunction). The planet passes from the evening to the morning sky. Every 116 days for Mercury. Every 584 days for Venus. Transits are rare — the alignment is rarely exact. Why? 3 / 27 M ERCURY T RANSIT C ONDITIONS Inferior conjunction must occur close to a node of Mercurys orbit for a transit to be visible. Transits are rare: just 14 between 2000 and 2100. 4 / 27 V ENUS T RANSIT C ONDITIONS Inferior conjunction must occur close to a node of Venus’s orbit for a transit to be visible. Transits occur in pairs separated by 8 years, with 105 years or 122 years between successive pairs. 5 / 27 P REDICTING TRANSITS Kepler studied the planets and derived three laws of planetary motion (1609 — 1619). Compiled the Rudolphine Tables (1627) contained a star catalogue and showed how to calculate the positions of the planets Keplers scientific ideas were not widely accepted in his lifetime. However, his methods predicted transits of Mercury and Venus in 1631. If the transits could be observed — it would be strong support for Kepler’s Laws. Johannes Kepler (1571 — 1630). 7 / 27 M ERCURY TRANSIT (1631) Mercury transit was predicted for November 7th 1631. The transit was successfully observed by Pierre Gassendi. Venus transit was predicted to happen in December 1631. Gassendi watched and waited for it — but it wasn’t seen. The Venus transit actually occurred but during night-time in Europe. Pierre Gassendi (1592 — 1655). 8 / 27 V ENUS TRANSIT (1639) William Crabtree (1610 — 1644) observing the Venus transit in 1639. Keplers data had predicted a near miss for Venus in 1639. English astronomers Jeremiah Horrocks and his friend William Crabtree refined the calculations; a transit would occur in 1639. Horrocks (in Much Hoole, Lancashire) and Crabtree (in Broughton, near Manchester) observed the start of the transit at sunset. 9 / 27 Horrocks’ observations of the transit allowed him to estimate the Sun-Earth distance; his figure of 60 million miles was 7 times bigger than astronomers had previously supposed. 10 / 27 K EPLER ’ S T HIRD L AW Kepler’s third law of planetary motion (the Harmonic Law) describes a relationship between the orbital period of the planet and its distance from the Sun. T2 ∝ D3 where T is the orbital period (in years) and D is the planet distance from the Sun (in terms of the Earth-Sun distance). By measuring the period astronomers could calculate the distance — relative to our distance from Sun. But astronomers had no direct way of measuring the Earth-Sun distance. Transits of Mercury and Venus could potentially give the answer. 11 / 27 T HE SIZE OF THE SOLAR SYSTEM In 1716 Edmund Halley (1656 — 1742) came up with a method to calculate the Sun-Earth distance using transit timings. Venus (or Mercury) follows a different path across the Sun seen from from widely spaced latitudes on Earth. Transit timings would allow the calculation of the actual distance. Venus, being much closer to Earth, is easier to measure. 12 / 27 V ENUS TRANSITS (1761 AND 1769) Many expeditions to far flung locations to record timings for these transits. Astronomers were granted safe passage in 1761 despite the Seven Years War between England and France. Captain James Cook led an expedition to Tahiti in 1769. Hundreds of measurements were made. James Cook (1728 — 1779). 13 / 27 T RANSIT OBSERVATIONS (1769) Timings from the Venus transits badly affected by a so-called black drop effect. However, data from both transits gave an Earth-Sun distance of 153 million km (with an uncertainty of ś1 million km). The absolute size of the solar system — distance to the Sun and planets in miles (or km) become known. 14 / 27 T HE B LACK D ROP E FFECT Venus and Mercury both show this effect Therefore, not caused by the atmosphere of the transiting planet. Bad seeing conditions in our atmosphere? Blurring of a high contrast image? Diffraction effects in the telescope? Black drop not widely observed with the 2004 transit of Venus. Satellite images show this effect for transits of Mercury. 15 / 27 T HE ATMOSPHERE OF V ENUS During the 1761 transit Mikhail Lomonosov (1711 — 1765) observed a ring of light around the planet Venus which he attributed to sunlight being refracted through the Venusian atmosphere. 16 / 27 V ENUS TRANSITS (1874 AND 1882) Sun-Earth distance refined to 149.6 million km (Simon Newcomb) using data from all previous transits. First photographs of a Venus transit. Parallax measurements were superseded by more accurate methods (radar reflection, Mars parallax). 17 / 27 V ENUS TRANSITS (2004 AND 2012) New focus: refine techniques for finding planets around sun-like stars. Most widely observed transit to date! VT-2004 project had nearly 3,000 participants around the world. Many school children made timings of transit. The AU measured to within a fraction of the accepted value. No more Venus transits until 22nd century... 18 / 27 19 / 27 20 / 27 O BSERVING THE M ERCURY TRANSIT It is dangerous to look at the Sun. Mercury is very small...optical aid essential. Binoculars and telescopes must be properly filtered. Homemade filters: take care constructing and securing housings for the filter. 21 / 27 T RANSIT TIMES FOR 09-M AY-2016 Transit times will vary with latitude. The following times are accurate for Northumberland: First contact: 12.12pm Second contact: 12:15 Mid transit: 3:57pm Third contact: 7:37pm Fourth contact: 7:40pm These timings are local (BST). Observing safely (but without dignity). 23 / 27 W ILL IT BE SUNNY ? Maybe...! 24 / 27 F UTURE TRANSITS Some future transits of Mercury... Date Visible from UK? 2016 May 09 2019 Nov 11 2032 Nov 13 2039 Nov 07 Yes. Partly; transit begins before sunset. Partly; transit in progress at sunrise. Partly; transit in progress at sunrise. Some future transits of Venus... Date Visible from UK? 2117 Dec 11 2125 Dec 08 2247 Jun 11 2255 Jun 09 No. Partly; transit in progress at sunrise. Yes. Partly; transit in progress at sunrise. 25 / 27 E ARTH TRANSITS ( FROM M ARS ) Transit of the Earth (and moon) as seen from Mars! This will happen on 10-Nov-2084. 26 / 27 The End Thank you :-) Dr Adrian Jannetta MIMA CMath FRAS