Student Features

2004 Transit of Venus
03.04.04
screen capture of Eclipse logo and banner

The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible. On average, there are 13 transits of Mercury each century. In contrast, transits of Venus occur in pairs with more than a century separating each pair.

No living person has seen a transit of Venus because the most recent one occurred in 1882. This situation is about to change since Venus will transit the Sun on Tuesday, 2004 June 08. The entire event will be widely visible from the Europe, Africa and Asia as shown in the map in Figure 1 (Low Res or High Res). Japan, Indonesia, the Philippines and Australia will witness the beginning of the transit but the Sun will set before the event ends. Similarly, observers in western Africa, eastern North America, the Caribbean and most of South America will see the end of the event since the transit will already be in progress at sunrise from those locations.

The principal events occurring during a transit are conveniently characterized by contacts, analogous to the contacts of an annular solar eclipse. The transit begins with contact I, the instant the planet's disk is externally tangent with the Sun. Shortly after contact I, the planet can be seen as a small notch along the solar limb. The entire disk of the planet is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, the silhouetted planet slowly traverses the brilliant solar disk. At contact III, the planet reaches the opposite limb and once again is internally tangent with the Sun. Finally, the transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Position angles for Venus at each contact are measured counterclockwise from the north point on the Sun's disk.

Greatest transit is the instant when Venus passes closest to the Sun's center (i.e. - minimum separation). During the 2004 transit, Venus's minimum separation from the Sun is 627 arc-seconds. The position angle is defined as the direction of Venus with respect to the center of the Sun's disk, measured counterclockwise from the celestial north point on the Sun.

Table 1
Geocentric Phases of the 2004 Transit of Venus
Event
Universal
Time
Position
Angle
Contact I
05:13:29
116°
Contact II
05:32:55
119°
Greatest
08:19:44
166°
Contact III
11:06:33
213°
Contact IV
11:25:59
216°

The table above gives the times of major events during the transit. Greatest transit is the instant when Venus passes closest to the Sun's center (i.e. - minimum separation). During the 2004 transit, Venus's minimum separation from the Sun is 627 arc-seconds. The position angle is defined as the direction of Venus with respect to the center of the Sun's disk, measured counterclockwise from the celestial north point on the Sun. Figure 2 (Low Res or High Res) shows the path of Venus across the Sun's disk and the scale gives the Universal Time of Venus's position at any instant during the transit. The celestial coordinates of the Sun and Venus are provided at greatest transit as well as the times of the major contacts.

Note that these times are for an observer at Earth's center. The actual contact times for any given observer may differ by up to ± 7 minutes. This is due to effects of parallax since Venus's 58 arc-second diameter disk may be shifted up to 30 arc-seconds from its geocentric coordinates depending on the observer's exact position on Earth. Table 2 and Table 3 list predicted contact times and corresponding altitudes of the Sun for locations throughout Canada and the United States, respectively. Table 4 provides transit predictions for a number of major cities around the world.

Recently (2004 Jan 22), new tables have been produced which cover transit circumstances for over a thousand cities. See: Transit Contact Times for Cities Around the World

Observing the Transit
Since the apparent diameter of Venus is nearly 1 arc-minute, it should be possible to see without optical magnification (but using solar filter protection) as it crosses the Sun. Nevertheless, the planet appears to be only 1/32 of the Sun's apparent diameter so a pair of binoculars or a small telescope at modest power will offer a much more satisfying view. Naturally, all binoculars and telescopes must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for observing a transit are identical to those for solar viewing. Amateurs can make a scientific contribution by timing the four contacts at ingress and egress. Observing techniques and equipment are similar to those used for lunar occultations. Since poor seeing often increases the uncertainty in contact timings, an estimate of the possible error associated with each timing should be included. Transit timings and geographic coordinates of the observing site (measured with a topographic map or GPS receiver) should be sent to: A. L. P. O. Mercury/Venus Transit Section, P.O. Box 16131, San Francisco, CA 94116, USA. The European Southern Observatory (ESO) is organizing a network of amateur astronomers and students to measure Earth's distance from the Sun during the transit. For more information, see their web site at:

http://www.eso.org/outreach/eduoff/vt-2004/

White light observations of contacts I and IV are not technically possible since Venus is only visible after contact I and before contact IV. However, if Hydrogen-alpha filtration is available, the planet will be visible against either prominences or the chromosphere before and after contacts I and IV, respectively. Observations of contacts II and III also require amplification. They are defined as the two instants when the planet appears internally tangent to the Sun. However, just before contact II, the so-called black drop effect is seen. At that time, the transiting planet seems to be attached to the Sun's limb by a thin column or thread. When the thread breaks and the planet is completely surrounded by sunlight, this marks the true instant of contact II. Contact III occurs in exactly the reverse order. Atmospheric seeing often makes it difficult to measure contact timings with a precision better than several seconds.


For more information on the Transit of Venus and this article, please visit:
http://sunearth.gsfc.nasa.gov/eclipse/OH/transit04.html


By Fred Espenak as Published in Observer's Handbook 2004, Royal Astronomical Society of Canada