On Tuesday, the 8th of June, 2004, the planet Venus will make a "transit" across the face of the sun.  This rare astronomical event can readily be observed with nothing more than an inexpensive pair of solar glasses; no telescope, binoculars or other special equipment is needed (though, of course, those can provide an even better view, as long as a solar filter is also used properly).

A planetary transit is possible only for Mercury and Venus, whose orbits are closer to the sun than the Earth's orbit.  A transit is somewhat similar to a solar eclipse, in that the planet moves into a position directly between the Earth and the sun.  But there is also an important difference: during a total solar eclipse the moon covers the entire face of the sun, blocking its light entirely, so that even during the daytime darkness falls on the earth; but the same phenomenon does not occur during a planetary transit.  Although Mercury and Venus are physically larger than the Earth's moon, they are also much farther away.  Thus, their "apparent size" as seen from the Earth -- that is, the amount of area they cover in the sky -- is quite a bit smaller than that of the moon.  In fact, when observed at night time, a planet normally appears to be a bright star -- nothing more than a point of light in the sky.  So when one of these planets passes directly between the sun and the Earth, it covers only a small part of the sun's surface.  Viewed through a suitable solar filter, the planet appears as a small dark circle within the larger disc of the bright sun.  (See diagram below.)

Transits of Venus are extremely rare.  Only six have occurred since the invention of the telescope in the early 1600's.  The last one was in 1882.  Two will occur during our lifetimes: on the 8th of June 2004 and the 6th of June 2012.  After that the next one will be in the year 2117.

Need more reasons to watch?  Read my article entitled "Why Astronomy?"

Additional information about the Venus transit, including solar diagrams, visibility maps and transit times for cities around the world, is available on the NASA web site at:

To view the transit, you will need two things: (1) you will need to know exactly when it will be visible from your geographical location, and (2) you will need a pair of solar viewing glasses.

You will not need a telescope or other magnifying equipment.  According to Fred Espenak of NASA:

Since Venus will subtend 61 arc-seconds, it should be visible to the naked eye using suitable filtration.

(Fred Espenak, NASA/GSFC; Planetary Transits Across the Sun)

For those locations in which the entire transit is visible from beginning to end (India, east Africa, eastern Europe and the Middle East), it will take about six hours for Venus to cross the face of the sun.  In other places the transit will begin before sunrise (west Africa and the eastern parts of North and South America) or end after sunset (Australia, Japan and east Asia), so that the window of opportunity for viewing it will be less than the full six hours.  And, unfortunately, in some parts of the world (the western parts of North and South America) the transit will occur entirely at night time so that none of it will be visible.

When the transit begins, Venus will appear as a black dot on the left edge of the sun, at the location marked with the number 1 on the solar diagram below.  It will be 3:07 pm on the 8th of June in Sydney, Australia.  At this time it will be 10:07 pm on the 7th of June in California, just after midnight in St. Louis, and 1:07 am on the 8th of June in Detroit and New York City, so the beginning of the transit will not be visible from those cities.

Sydney viewers will be able to watch the first hour and 45 minutes of the transit until sunset, when Venus will have reached the spot marked #2 on the diagram.  Between #2 and #3 it will be night time in eastern Australia and throughout most of North and South America.

When the sun rises in New York City on the 8th of June, the transit will still be in progress, with Venus at position #3.  A half-hour later the sun will rise in Detroit, and Venus will have progressed further to the right at #4.  The sun will rise 40 minutes later in St. Louis (although the local time will be earlier because it is in a different time zone).  At this time the transit will be nearing completion, with Venus at #5.

Viewers throughout eastern America will be able to continue watching the transit until Venus reaches the bottom right edge of the sun at #6.  However, this will occur before the sun rises in the western parts of North and South America, so none of the transit will be visible from California or the other western states.

In the table below I have listed viewing times for a few selected cities.  Numbers in the table are keyed to locations on the solar diagram to the right.  If your city is not shown, you may consult the more comprehensive listing on the NASA web site given above.

Solar viewing glasses allow less than 0.001% of the sun's light to reach your eyes.  This is a level that is safe for extended viewing.  Note, however, that solar viewing glasses are not designed for use with cameras, binoculars or telescopes.  If you are interested in using a solar filter with a camera or telescope, you should get something that is designed specifically for that purpose.  Be sure that the filter covers the outside lens -- the one facing the sun -- rather than the inside lens, facing your eye.

Makeshift filters using smoked glass, exposed film, CD-ROMs or other materials do not provide adequate protection because they do not filter enough of the sun's harmful ultraviolet and infrared radiation, which can damage the retina of your eye.

Extensive tests have been performed on the safety of various materials for solar viewing.  For more information see the following article.

Solar viewing glasses can be obtained from a number of sources.  I purchased a supply for the December 2002 solar eclipse, and some of these are still available.  If you live in Sydney and are interested in buying a pair from me, please let me know.

Solar glasses can also be purchased from:

The price quoted to me on the phone today was $6.50 (Australian dollars).

If you live elsewhere, you might try a local telescope shop, or you can visit the following web sites for more information about ordering solar viewers:

Finally, be sure to set your alarm clock on the day so you don't forget.  Happy transit viewing!