"Long before it's in the papers"
June 03, 2013

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Rare Venus event could aid planet-hunters, delight sky-watchers

May 1, 2012
Courtesy of the Institute of Physics, U.K.
and World Science staff

Mil­lions of peo­ple around the world will be able to see Ve­nus pass across the face of the Sun next month, in an event not to re­cur un­til 2117.

Sky-gazers are be­ing urged to learn cru­cial sun-view­ing safe­ty pre­cau­tions—the same ones re­quired for eclipses—be­fore at­tempt­ing to watch the event, on June 5 and 6. Ve­nus will first ap­pear as a small black dot on the Sun’s sur­face, then spend about six hours cross­ing.

A Ve­nus trans­it as seen by NA­SA's Sun-observing TRACE space­craft in 2004. (Cour­te­sy NA­SA)


The event could of­fer new in­forma­t­ion to as­tro­no­mers hunt­ing for un­known plan­ets. That’s be­cause an in­creas­ingly com­mon meth­od of find­ing them is to scan the night sky for oc­cur­rences like the Ve­nus cross­ing, but much fur­ther off.

In this mon­th’s is­sue of the mag­a­zine Phys­ics World, Jay Pasa­choff, an as­tron­o­mer at Wil­liams Col­lege, Mas­sa­chu­setts, ex­plores the sci­ence and history be­hind Ve­nus trans­its.

These events occur when Ve­nus and Earth are in line with the Sun. The rest of the time Ve­nus is seen to pass be­low or above the Sun be­cause the or­bits of Ve­nus and Earth are slightly askew from one an­oth­er. Ve­nus trans­its oc­cur in pairs, in which eight years sep­a­rate one trans­it from the oth­er. Af­ter each such pair, more than a cen­tu­ry pas­ses be­fore the next pair. The June event will be the sec­ond of its pair, the first hav­ing oc­curred in 2004.

Build­ing on the­o­ries of Nic­o­la­us Co­per­ni­cus from 1543, sci­en­tists were able to pre­dict and rec­ord the trans­its of both Mer­cu­ry and Ve­nus in the cen­turies that fol­lowed, Pasa­choff notes. Jo­han­nes Kep­ler suc­cess­fully pre­dicted that both plan­ets would trans­it the Sun in 1631, part of which was ver­i­fied with Mer­cu­ry’s trans­it of that year. But the first Ve­nus trans­it to ac­tu­ally be viewed was in 1639 – an event pre­dicted by the Eng­lish as­tron­o­mer Jer­e­mi­ah Hor­rocks. He ob­served the trans­it in the vil­lage of Much Hoole in Lan­ca­shire – the only oth­er per­son to see it be­ing his cor­re­spond­ent, Wil­liam Crab­tree, in Man­ches­ter.

Lat­er, in 1716, Ed­mond Hal­ley pro­posed us­ing a trans­it of Ve­nus to pre­dict the pre­cise dis­tance be­tween the Earth and the Sun, known as the as­tro­nom­i­cal un­it. As a re­sult, hun­dreds of ex­pe­di­tions were sent all over the world to ob­serve the 1761 and 1769 trans­its. A young James Cook took the En­deav­our to the is­land of Ta­hi­ti, where he suc­cess­fully ob­served the trans­it at a site that is still called Point Ve­nus.

Pasa­choff ex­pects the trans­it to con­firm his team’s the­o­ry about the phe­nom­e­non called “the black-drop ef­fect” – a strange, dark band link­ing Ve­nus’s sil­hou­ette with the sky out­side the Sun that ap­pears for about a min­ute start­ing just as Ve­nus first en­ters the so­lar disk.

Pasa­choff and his col­leagues will con­cen­trate on ob­serv­ing Ve­nus’s at­mos­phere as it ap­pears when Ve­nus is only half on­to the so­lar disk. He al­so be­lieves that ob­serva­t­ions of the trans­it will help as­tro­no­mers who are look­ing for plan­ets or­bit­ing stars oth­er than the Sun.

“Do­ing so ver­i­fies that the tech­niques for stu­dy­ing events on and around oth­er stars hold true in our own back­yard. In oth­er words, by look­ing up close at trans­its in our so­lar sys­tem, we may be able to see sub­tle ef­fects that can help ex­o­plan­et hunters ex­plain what they are see­ing when they view dis­tant suns,” Pasa­choff writes. An ex­o­plan­et is a plan­et out­side our so­lar sys­tem.

Not con­tent with view­ing this year’s trans­it from Earth, sci­en­tists in France plan to use the Hub­ble Space Tel­e­scope to ob­serve the ef­fect of Ve­nus’s trans­it very slightly dark­en­ing the Moon. Pasa­choff and col­leagues even hope to use Hub­ble to watch Ve­nus pas­sing in front of the Sun as seen from Ju­pi­ter, on Sept. 20, and to use NASA’s Cas­si­ni space­craft, which or­bits Sat­urn, to see a trans­it of Ve­nus from Sat­urn on Dec. 21.


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Millions of people around the world will be able to see Venus pass across the face of the Sun next month, in an event not to recur until 2117. Sky-gazers are being urged to learn crucial sun-viewing safety precautions—the same ones required for eclipses—before attempting to watch the event, on June 5 and 6. Venus will first appear as a small black dot on the Sun’s surface, then spend about six hours crossing. The event could offer new information to astronomers hunting for unknown planets. That’s because an increasingly common method of finding them is to scan the night sky for occurrrences like the Venus crossing, but much further off. In this month’s issue of the magazine Physics World, Jay Pasachoff, an astronomer at Williams College, Massachusetts, explores the science behind Venus’s transit and gives an account of its history. Venus transits take place when Venus and the Earth are in line with the Sun. The rest of the time Venus passes below or above the Sun because the orbits of Venus and Earth are slightly askew from one another. Venus transits occur in pairs, in which eight years separate each transit from each other. After each such pair, more than a century passes before the next pair. The June event will be the second of its pair, the first having occurred in 2004. Building on the original theories of Nicolaus Copernicus from 1543, scientists were able to predict and record the transits of both Mercury and Venus in the centuries that followed, Pasachoff notes. Johannes Kepler successfully predicted that both planets would transit the Sun in 1631, part of which was verified with Mercury’s transit of that year. But the first Venus transit to actually be viewed was in 1639 – an event predicted by the English astronomer Jeremiah Horrocks. He observed the transit in the village of Much Hoole in Lancashire – the only other person to see it being his correspondent, William Crabtree, in Manchester. Later, in 1716, Edmond Halley proposed using a transit of Venus to predict the precise distance between the Earth and the Sun, known as the astronomical unit. As a result, hundreds of expeditions were sent all over the world to observe the 1761 and 1769 transits. A young James Cook took the Endeavour to the island of Tahiti, where he successfully observed the transit at a site that is still called Point Venus. Pasachoff expects the transit to confirm his team’s theory about the phenomenon called “the black-drop effect” – a strange, dark band linking Venus’s silhouette with the sky outside the Sun that appears for about a minute starting just as Venus first enters the solar disk. Pasachoff and his colleagues will concentrate on observing Venus’s atmosphere as it appears when Venus is only half onto the solar disk. He also believes that observations of the transit will help astronomers who are looking for planets orbiting stars other than the Sun. “Doing so verifies that the techniques for studying events on and around other stars hold true in our own backyard. In other words, by looking up close at transits in our solar system, we may be able to see subtle effects that can help exoplanet hunters explain what they are seeing when they view distant suns,” Pasachoff writes. An exoplanet is a planet outside our solar system. Not content with viewing this year’s transit from Earth, scientists in France plan to use the Hubble Space Telescope to observe the effect of Venus’s transit very slightly darkening the Moon. Pasachoff and colleagues even hope to use Hubble to watch Venus passing in front of the Sun as seen from Jupiter – an event that will take place on 20 September this year – and will be using NASA’s Cassini spacecraft, which is orbiting Saturn, to see a transit of Venus from Saturn on 21 December. “We are fortunate in that we are truly living in a golden period of planetary transits and it is one of which I hope astronomers can take full advantage,” he wrote.