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Once (at most twice) in Your Life — if You're lucky…

…you can watch one of the rare transits of Venus before the Sun. Therefore it was clear, that I took a vacation for that legendary event. Jump directly to the report

Why these are that Rare?

Every 584 days (about one year and 7 months) Venus happens to be in lower conjunction with the Sun. But the plane of Venus' motion is tilted more than three degrees against that of the Earth; while this seems to be not much, it has dramatical consequences: first Venus is away just about 40 millions km (about 22 million nautical miles) from Earth (varies slightly with the distance of Earth from Sun; the Venus orbit is a nearly perfect circle on the other hand) during that constellation, what means barely more than one-fourth (!) of the distance Earth from Sun (at average nearly 150 millions km or about 80 million nautical miles). Second the apparent solar diameter is just above half a degree. And I can tell, that I have seen at an earlier occasion Venus during a lower conjunction nearly 10 degrees away from Sun at daylight without any trouble (Sun is even for less than perfect mounted telescopes a good mark to find Venus, but be careful!). And that is a typical event; that Venus is less than a quarter of a degree away from Earths plane (what means in immediate vicinity of one of the two nodes of her orbit with Earths plane) and not 5 or more degrees, is rather uncommon therefore.

Since the midth of the second millennium and up to the end of the current one Venus transits happen to take place in pairs, seperated eight years, after a long gap of about 120 years. They are — opposed to total solar eclipses! — like total lunar eclipses visible from the entire half of Earth, from which the Sun respective the Moon are above the horizon. Because of this in most cases you will have exactly once the chance, to see such a transit in your lifetime without voyage (acceptable obversation conditions presumed!), but you may fail though even when living long: a grandmother of mine was born a number of years after the last Venus transit 1882, but died already many years before the currently last in 2004, though she lived for about 90 years.

The Parallax of Venus and Sun

With the contact times (first contact: disks of Venus and Sun touch from the outer side at the begin, second contact: from the inner side, third contact from inside again before exiting, fourth contact from the outside when exiting) measured from different locations of the Venus observable half of Earth it is possible, to establich the Venus and Solar parallax, what means the real distance in km (or maybe nautical miles, if you like). This and the fact, that no planet is approaching Earth that much like Venus, caused some great trials in history, to use these events to get the dimensions of Earths orbit (and with the Kepler laws of planetary motion the dimensions of the Solar System). But there are several issues regarding measurement of those contact times: the dimmering of the rim of the Sun, the air turbulence, the refraction caused limited sharpness of the Venus image, the atmosphere of Venus… (the so-called black drop makes determining the contact times really troublesome; some kind of bridge between the disks of both stellar objects) That's the reason, that after the fifth Venus transit observed in 1882 other methods were chosen, especially with better precision of itself. Mercury and Mars were less well suited, but the minor planet Eros was used because of it's sometimes even lower distance with the same method, but since the 60's of the twentieth century radar echoing the Venus (again!) has substituted these older techniques, ironically Venus is the preferred target because of the low distance and appreciable size again. The results are so good because of our high-precision time measurements and limited by the undulations of Venus' surface to just centimeters (inchs)!

The Venus Transits of Human History

You need at least solar eclipse glasses or the like to see the small, but no way zero dimensional looking dot of Venus in front of Sun (even an arc minute, what means big enough for human eyes without magnification tricks); and it's necessary to be capable of calculating the than not visible Venus position during conjunction. With his outstanding planet position tables Johannes Kepler determined a Venus transit for 1631, one year later — after his death. But the two only observers trying to put his prediction into work missed the transit unluckily, which had a much shorter trajectory than in 2004 over the Sun, due to poor weather conditions and non-ideal observation locations.

Eight years later a young Englishman with name Horrocks calculated also shortly before the next transit 1639 that one with sufficient precision, so he and his friend Crabtree were able to follow it at least partly. That was the first Venus transit at all, which was ever seen by humans (at least so far we can prove it; but it seems unlikely, that anybody else had seen such one before).

During the 18. century as well in 1761 as in 1769 big expeditions were carried out (see above, Solar System size determination!). The second human observed transit in 1761 was a little unlucky in several regards: war conditions and poor weather prohitited a big wealth of results sufficient for precise calculations. 1769 the conditions were much better and the issues from 1761 were learned from too. The famous trip of James Cook with the endeavor in 1769 served the sole purpose to get good measurements of the Venus transit! But the black drop phenomenon averted very precise measurements; Cooks diverse observer groups got contact times differing up to 20 seconds though they watched it from the virtually same location.

The transits four and five (1874 and 1882) of human history saw modern kind international observation campaigns with hundreds of professional investigators. Either way the results had still rather limited precision; more was simply not possible with that method; a few fairly good photographs of the beginning of that technique were taken in 1882 during the fifth transit.

Perspective: the Venus transits no. 6 (2004) and 7 (2012) are “only” just fun events, maybe aside from eventual Venus atmosphere investigations as measurements and calibrations for extrasolar planet hunts by transit observation (the solar radiation is reduced by about one per millenium from Venus — small solar eclipse —, what can't be recognized, but it is detectable with modern equipment). The following report deals with this sixth observed event in 2004.

The Venus Transit of 8. June 2004 in Germany

Prelude

The planet Mercury moves on a much closer and faster orbit around the Sun as Venus does; despite it's bigger orbit plane tilt (about seven opposed to three degrees) it's average apparent seperation from the Sun is no bigger than that of Venus: because it comes not nearly that close then Venus.

Last year I grabbed my chance to observe my first Mercury transit (happens about 13 times per century, if you're interested, no big deal therefore), which happened very much like this Venus transit: at begin of June, starting shortly after sunrise, enduring nearly as long.

Of course Mercury is much smaller and no way visible with solar eclipse glasses alone (can be seen only with telescope aid) and not that prominent opposed to sunspots. At least the procedure of 2003 worked so well, that we sticked to it for the Venus transit too, this time additionally imaging it. The projection approach is ideally suited for this kind of event: comfortable and for several observers at the same time using the same equipment it gives a fine impression of this “cosmic shadow game”.

We used a Newton reflector with 114 mm mirror diameter (4.5 inch) and a focus length of 900 mm (354 inch = 2.95 foot) and oculars of focal length 20 resp. 40 mm for the projection; the pictures were taken from the white piece of carton with a digital cam Canon Powershot A70. Sadly the dirt on the optical surfaces of the telescope (primary and secondary mirror as the ocular lenses) proved as a little disturbing during the observations; an issue, which is barely recognizable at night time.

Location was the plain roof of a house, to be able to see the event in full length, despite the Sun had rather low altitude when the Venus disk entered it.

The weather developed exactly as predicted by my science buddies of the meteorology profession: at Monday (the 7. June 2004) before the Venus transit number six (due to our — too much? — anthropogenic enumeration) the conditions arose, which we call summer here in Germany; and even two days after the event at lunch time of the 10. June 2004 the weather was dry and sunny. Ideal preconditions…

The Morning of the 8. June 2004

7:00. The telescope is quickly carried to the top, we seek and find also rather fast the location, from which we saw the Mercury transit the year before so well.

The still low hanging Sun is smeared accordingly by strong air turbulence; an issue absent last year — because there was high fog, so that we missed the first two contacts, i.e. the entering of the Mercury disk on Suns disk, then completely.

7:20. The first contact happens not really observed, as expected, you have to know the location of Suns disk rather precise, for not missing it. Because we didn't plan measurements (of contact times), that is not important.

7:22. We see it! The rim of Venus has created a little edge at the Suns rim, now we follow interested the twenty minutes long coming in of Venus' disk onto the Suns one between contacts one and two — though we missed the very first minutes.

Slowly the true dimensions of the nearly Earth sized planet are revealed (around 12100 km (6530 nautical miles) diameter, i.e. only about 5% less than Earth). In projection the disk takes a considerable area, about 1/30 of the Suns apparent diameter.

7.39. The second contact: this can be easily observed, of course, after we followed the coming in of the Venus; but the well-known (not to say infamous) phenomenon of the black drop prevents as expected close comparison with the calculated time. All seems to be somewhat diffuse…

After second contact it takes nearly five and a half hour, until the third contact happens — enough time for extended extra activity like glimpses with the solar eclipse glasses, in which Venus can be well seen as dark, not really dot in front of Suns disk, for changing oculars and trials to photograph the projection. The Sun and also Venus image becomes clearer, because the air turbulence decreases slowly with the much bigger height, despite the temperature rises recognizably too.

By the way only two rather tiny sunspots (not even at Mercurys size, I tend to say!) were visible, which couldn't distract away from the event at all — the solar activity is now near the minimum after the surprising short revival last year.

It proved useful, to hold the projection carton from time to time in front of the telescope aperture, to reduce the air turbulence in the telescope, induced by Suns insolation; afterwards we had again a clearer (more stable) image.

13:04. The third contact: the tense is increasing now, after the Venus recently approached quickly the Suns rim on her short path over Suns disk. Again the shadow of Venus fuses well before the true contact with Suns rim via the black bridge, but either way it is much better to see, now when the Sun is more than 60 degrees high in the sky (for central Europeans this looks like immediate vicinity of the zenith).

Rather quickly the Venus disk exits now the Suns again, especially at the end this happens rather fast, because the area loss is even stronger.

13:20–13:13. The fourth contact is barely visible again, because the rim is not very well defined at high magnification and the air is shivering: though we can observe the exit nearly to the end.

Here are some pictures resp. parts of it with time, all images were taken by Helmut Urbat of the carton I held…

Venus before Sun, cut8:04 UT = 10:04 CEST, just before midth of transit

Venus before Sun, cut9:34 UT = 11:34 CEST, after midth of transit

Venus before Sun, cut11:07 UT = 13:07 CEST, after third contact

Venus before Sun, cut11:08 UT = 13:08 CEST, after 3. contact, complete of this cut

Venus before Sun, cut11:10 UT = 13:10 CEST, after 3. contact

Venus before Sun, cut11:15 UT = 13:15 CEST, before 4. contact

Venus before Sonne, Ausschnitt11:20 UT = 13:20 CEST, before 4. contact

Venus before Sonne, cut11:22 UT = 13:22 CEST, about 1.5 minutes before 4. contact

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remarks etc. to: stefan.urbat@apastron.lb.shuttle.de public key

(URL: http://www.lb.shuttle.de/apastron/venusTr_en.html)

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