On June 5, 2012, 22:09 UT, the Sun, Venus and the Earth will be in alignment and observers on Earth where the Sun is visible at this time will view a ‚Transit of Venus,‚ a rare occurrence as only seven transits have occurred since the invention of the telescope.
An astronomical transit occurs when a smaller closer celestial object passes across the face (disc) of a larger more distant celestial object. In this case, the time taken for Venus to transit the Sun will be six hours and forty minutes. Viewed from Earth, transits of the Sun can only occur with inner planets, those that have an orbit closer to the Sun than ours, namely, Mercury and Venus. In order to see the transit, the Earth, transiting planet and Sun have to be aligned. Transits of Venus across the Sun are extremely rare, in fact, only seven such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882, 2004). This event will be the first transit while there is a spacecraft orbiting that planet ‚ ESA’s Venus Express. Transits of Venus are only possible during early December and early June when Venus’s orbit crosses the plane of the ecliptic (the plane of the Earth’s orbit around the Sun). If the Sun, Venus and Earth form a straight line when this occurs (termed inferior conjunction) a transit will occur. Venus transits occur at intervals of 8, 121.5, 8 and 105.5 years. The last Venus transit was in 2004, eight years ago, and so unfortunately the next transit will not occur within our lifetime. The next two are predicted to occur on December 11, 2117 and December 8, 2125! Unfortunately, the transit will not be visible from South Africa as it occurs between 00:09 and 06:49 local time on June 6 (6 hours 40 minutes in total). However, if we were able to see the transit what could we expect? Astronomers describe the four main phases of a transit as follows:
The ‚Greatest transit‚ is the instant at which Venus is in the middle of its path across the solar disc, this marks the halfway point in the timing of the transit.
Figure 1: Image from http://eclipse.gsfc.nasa.gov/OH/transit12.html This diagram shows the path that Venus will take across the Sun on June 6 (as well as the path it took in 2004). The four contact points are marked along with the point of greatest transit. The numbers on the scale give the time (UT) at which Venus is at a particular location as it crosses the Sun. The times for each of the contact points for the June transit are shown in Figure 1. Note that these times are for an observer at Earth’s centre. The actual contact times for any given observer may differ by up to ¬±7 minutes. This is due to effects of parallax, as Venus’s 58 arc-second diameter disc may be shifted by up to 30 arc-seconds from its geocentric (Earth centred) coordinates depending on the observer’s exact position on the surface of the Earth. During the 2012 transit, Venus’s minimum separation from the centre of Sun will be 554 arc-seconds, it will not pass across the centre of the Sun but rather to the North of centre. Figure 2 shows the global visibility of the Transit of Venus of 2012. The un-shaded region shows where the entire transit is visible. The grey shaded region indicates where no part of the transit is visible and the blue and green shaded regions indicate where only part of the transit will be visible (Sun setting and rising respectively). Taken from http://eclipse.gsfc.nasa.gov/OH/ transit12.html Although not visible from South Africa, the entire transit will be visible from north western Canada, Alaska, the western Pacific, northern Asia, eastern Australia, and New Zealand. The Sun sets while the transit is still in progress from most of North America and northwest South America. Similarly, the transit is already in progress at sunrise for observers in central Asia, Europe, the Middle East and eastern Africa. No portion of the transit will be visible from western Africa, and much of South America. Note that due to the International Date Line the Western Hemisphere will see the transit on June 5. How to observe a Transit As the apparent diameter of Venus is nearly 1 arc-minute, it is just possible to see Venus crossing the Sun without a telescope. Note that special solar filter protection is required, never look directly at Sun with or without sunglasses, it can cause blindness. However, as the planet’s angular size is only 1/32 of the Sun’s apparent diameter, using binoculars or a small telescope will make the transit much clearer. All binoculars and telescopes must be equipped with adequate filters to ensure safe solar viewing. In fact, the safest way to watch the transit using a telescope is to project the image of the Sun onto a piece of card behind the telescope’s eyepiece.
White light observations of contacts I and IV are not technically possible, since Venus is only visible once it has entered the solar disc. However, if a Hydrogen-Alpha filter is used, it is possible to see the planet against either solar prominences or the solar chromosphere before and after contacts I and IV. Observations of contacts II and III also require magnification to see clearly. Just before contact II, the ‚black drop‚ effect is seen. Here, Venus appears attached to the limb of the Sun by a thin thread, forming a small black teardrop shape. This occurs as it fully enters the solar disc just after contact II and just before contact III as it begins to leave. The black drop is thought to be an optical effect caused by the effect of observing through Earth’s atmosphere, combined with diffraction of light inside the telescope, and by the dimming of the intensity of the Sun’s surface just inside its apparent outer edge. Atmospheric seeing and the black drop effect often make it difficult to measure contact timings to accuracies better than several seconds. Live webcast Many observatories around the world will be the streaming the event live across the internet. Some URLS are provided below:- keckobservatory.org/news/venus_transit_live_keck_observatory www.rssd.esa.int/index.php?project=VENUSEXPRESS&page=venus_transit – which shows images in both H-alpha and white light. venustransit.nasa.gov/transitofvenus The history of Transits and their uses in Astronomy Sir. Edmond Halley first realised that transits of Venus could be used to measure the Sun’s distance. The technique relied on comparing observations made from around the world. By timing how long it took for Venus to cross the solar disc from different locations on Earth, one could work out the path the planet appeared to take across the Sun’s disc from different locations. These straight lines, or ‚chords‚ are parallel to each other, but not exactly overlaying because of parallax effects. By measuring the angle of parallax, or the perpendicular distance between the two parallel paths and knowing the distance between the two observers on Earth it was possible to calculate the distance to Venus using trigonometry. From this, the distance to the Sun could be calculated it was known (from Kepler’s 3rd Law of planetary motion) that the ratio of the Venus-Sun and Earth-Sun distance was 0.72. (Or that the distance to Venus is 0.28 times the distance to the Sun). Venus transits were considered better suited to this goal than Mercury transits because Venus is closer to Earth and consequently exhibits a larger parallax. Halley challenged future generations to organise major expeditions in order to observe the transits of 1761 and 1769. Unfortunately, his method proved impractical since contact timings of the desired accuracy were impossible due to the effects of atmospheric seeing and diffraction (the black drop effect mentioned earlier). However, observations of the 1761 and 1769 transits of Venus gave astronomers their first good estimate for the Sun’s distance. Undeterred, another major observing campaign was mounted by many nations for the Venus transits of 1874 and 1882. Indeed, the Cape Astronomer Royal from 1879 to 1907, Sir. David Gill mounted an expedition to Mauritius to observe the 1874 Venus transit, and later when employed at the Royal Observatory, Cape of Good Hope (now home to the South African Astronomical Observatory). He conducted observations of the 1882 Venus transit at the observatory grounds in Cape Town. Additionally, a British Transit of Venus expedition was sent to Touwsriver, and an American expedition was sent to Wellington to observe the 1882 transit. Gill’s measurement of the distance to the Sun was accepted as the most accurate at that time. The distance to the Sun and planets can now be measured extremely accurately using radar. During the transit of 1761 astronomers noticed a halo of light around the planet’s circumference during ingress and egress, this is known as an aureola. This provided the first proof, that Venus has an atmosphere as the effect is caused by refraction of sunlight in the dense upper atmosphere of Venus. We now know Venus has an inhospitable dense atmosphere of carbon dioxide and nitrogen with clouds of sulphuric acid. Today, transit events are used to detect and study exoplanets, planets orbiting stars other than our Sun. As a planet passes in front of a star, it temporarily blocks out a tiny portion of the starlight, not only revealing its presence, but also providing clues about the planet’s size. Transits are also being used to search for exoplanets that may harbour life. If a planet has an atmosphere, a small fraction of the light from the star will pass through the atmosphere and reveal its properties, such as its chemical composition, including the presence of water.
The Sun Earth Moon System (SEMS) team is doing a live webcast of the venus transit from Anchorage, Alaska. You can view it here: http://www.sems.und.edu
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