By Steven van Roode
The diagram below, drawn to scale, shows how Venus will traverse the solar disk as seen from the centre of the earth. The five positions of Venus along the time-scale are the exterior (1) and interior (2) contacts at ingress, the least distance to the centre of the solar disk (m) and the interior (3) and exterior (4) contacts at egress. The position angle of Venus is measured counterclockwise along the solar limb from the direction of the north on the celestial sphere (N).
The geocentric circumstances for the 2012 transit are summarized in the following table. The times are in Universal Time.
|contact||time||position angle / separation|
|1. ingress, exterior||22:09:42||40.7|
|2. ingress, interior||22:27:30||38.2|
|3. egress, interior||04:31:44||292.7|
|4. egress, exterior||04:49:32||290.1|
At the time of the transit the sun measures 1891.38 seconds of arc across, while Venus is only 58.26 seconds of arc in diameter. This makes Venus just visible to the unaided eye as a small black dot. When fully on the sun’s disk, Venus covers only 0.1 percent of the sun, so you won’t notice any decrease of sunlight during the transit. Therefore, it is essential to use a safe solar filter! See the Six ways to observe the transit of Venus on this website for more information on viewing the transit of Venus.
Do you want this map in your own language? Go to eclipse-maps.com to download one.
The transit will be wholly visible from eastern Australia, New Zealand, New Guinea, the Philippines, China, Korea, Japan, the islands of the western Pacific Ocean, Hawaii, Russia, Alaska and north-west Canada. In the far north the entire transit can also be seen, since the sun does not set. From eastern Canada, the United States, Central America, the Caribbean and northwest South America only the first part of the transit will be visible, because first contact occurs in the afternoon of June 5 and the sun sets while the transit is still in progress. In western Asia, Europe, the Middle East and eastern Africa only the last part of the transit will be visible. On June 6 the sun will rise with Venus already on its face and last contact will be seen in the morning hours. A special case forms a triangular area from Iceland downwards. Although both ingress and egress will be visible, the transit won’t be visible in its entirety. Due to the short summer nights at these latitudes, the sun sets after ingress on June 5 and rises before egress on June 6, leaving only the middle part of the transit invisible!
Owing to the effects of parallax, arising from both the latitude of the place and its diurnal motion imparted by the earth’s rotation, the ingress and egress as seen from a point on the surface of the earth will occur a little earlier or later than when viewed from the centre of the earth. There are locations where a specific contact will be most accelerated or most retarded by the effects of parallax.
Because these locations are favourable for Delisle’s method of finding the sun’s distance, they are called Delislean poles. The sun is on the horizon at each contact listed below: setting at ingress and rising at egress.
|1. most accelerated||22.03.07||41.5||37°.6166 W||43°.6590 N|
|1. most retarded||22.16.20||39.8||140°.4070 E||45°.1330 S|
|2. most accelerated||22.20.43||39.1||40°.2196 W||45°.6760 N|
|2. most retarded||22.34.22||37.1||138°.0414 E||47°.3720 S|
|3. most accelerated||04.24.52||293.7||166°.2566 W||22°.0440 S|
|3. most retarded||04.38.30||291.6||11°.2411 E||20°.1750 N|
|4. most accelerated||04.42.53||291.0||169°.6079 W||19°.5860 S|
|4. most retared||04.56.06||289.2||7°.8442 E||17°.9830 N|
As a result, the apparent stay of Venus on the sun’s disk will also vary according to an observer’s location. There are two locations where the transit’s duration will arrive at its extreme values, but only one is in the area where the entire transit will be visible. These points are sometimes referred to as Halleyan poles, because the largest difference in duration is most favourable for finding the sun’s distance using Halley’s method. For the 2012 transit of Venus, the location where you might experience the shortest duration is positioned well in the area of visibility, near 38 degrees south and 170 degrees east, not far from the North Island of New Zealand. This makes the west coast of the North Island of New Zealand an excellent observing site insofar the determination of the solar parallax is concerned.
The other Halleyan pole, where the duration will be longest, is located at the opposite, near 38 degrees north and 10 degrees west, which is very close to Lisboa in Portugal. But the transit will not be visible from this location. The location on land within the area of visibility that will experience the longest duration is the south coast of Iceland, at about 63.5 degrees north and 18 degrees west. So, obervations from the south coast of Iceland, paired with observations from the west coast of the North Island of New Zealand, will yield an accurate value of the solar parallax using Halley’s method of durations. You can also download a world map, which shows the difference in duration in more detail.