Eclipses: English version - Venus Transit across the Sun, June 8,
2. Total Solar Eclipse, February 26, 1998,Venezuela
3. Total Solar Eclipse, August 11, 1999,Turkey
4. Total Solar Eclipse, June 21, 2001, Zambia
5. Total Solar Eclipse, March 29, 2006, Turkey
6. Total Solar Eclipse August 1, 2008, Siberia
7. Venus Transit across the Sun, June 8,
JUNE 8, 2004 TRANSIT OF VENUS ON THE SUN
The Transit of the planet Venus on the Sun is an event quite sporadic because rarely the planet crosses the line joining the Earth and the Sun (line of nodes) being its orbit inclined at 3.4 degrees with respect to the Earth. Thus, instead of taking place every 584 days, the Transits usually occur in pairs spaced about 8 years, but after we will have to wait for more than 100 years for the next pair. From the seventeenth century, when it was invented the telescope, there were up to now six Transit and precisely on 7 December 1631, 4 December 1639, 6 June 1761, 3 June 1769, 9 December 1874, and 6 December 1882. The current Transit of 8 June 2004 takes place 122 years after the last of 1882.
In 1677 the astronomer Halley proposed to use the transit of Venus on the Sun to measure its distance from the Earth with the method of parallax by observing the phenomenon from different points of the Earth. Because according the laws of Kepler, you knew all the ratio between the orbital distances of the planets of the solar system, this would have made it possible to determine the Astronomical unit (AU), i.e. the distance between the earth and the Sun, fundamental unit for the measurement of cosmic distances. Halley died in 1742 and was unable to attend to the passage of the 1761 for which they were organized several scientific expeditions, but weather conditions and other factors made unusable data collected. The transit of 1769 was observed from 76 different points of the globe and the most important observation was the one promoted by the Royal Academy of England by instructing the explorer James Cook, during his first trip around the world (1768-1771), to stop in Tahiti to observe the Transit. Starting in August 1768 from Plymouth to the command of the ship Endeavour, Cook double the Cape Horn and reached Tahiti in April 1769, two months in advance. The transit was observed and was noticed the phenomenon of black-drop that made it uncertain the determination of time of contact. The black-drop is an optical distortion of Venus shape, like a drop of water, before it touches the solar edge and its exact causes are still being studied. In conclusion, the observations of 1769 not allowed precise measurements, and only in 1800, with the aid of photography, the Sun-earth distance of about 153 million km was obtained, with an error of 2% in excess of the value known today with other methods.
Today the astronomical interest of the transit of Venus is limited; some researchers exploit the event to study indirectly the transit of an extrasolar planet in front of its star by measuring the variation in brightness of the sun. This variation is the same that you would expect for the transit of a planet's like Earth in front of its star, method now used in search for extrasolar planets. The variation in brightness is 0.01 % for planets the size of the Earth and the 1% for planets the size of Jupiter. In 2007, after launch of the mission Kepler by NASA, would be possible to observe 100000 stars and discover orbiting planets. Other measures will be of spectroscopic nature on the atmosphere of Venus backlit by the sun. The transit will be also observed from satellite to study the phenomenon of the black-drop out of the earth's atmosphere.
The transit of Venus of 8 June 2004 can be seen from across Europe and will last approximately 6 hours. The next transit will take place on June 6, 2012 but will not be fully visible in Europe, after you will have to wait for 105 years until the December 11, 2117, then there will be those of the 2125 and 2247.
TRANSIT OBSERVATION
The observation place is Rome and coordinates are:
Latitude: 41° 48' 0,0" Nord
Longitude: 12° 36' 0,0" East
Observation condition and times of Venus Transit are downloaded from web site NASA - 2004 Transit of Venus
(UT: Universal Time):
First Contact (external): 05:20:11 UT, 07:20:11 Local Time.
Sun Elevation: 17°
Second Contact (internal): 05:39:55 UT, 07:39:55 Local Time. Sun Elevation: 21°
Central position: 08:23:01 UT, 10:23:01 Local Time. Sun Elevation: 51°
Third Contact (internal): 11:04:33 UT, 13:04:33 Local Time. Sun Elevation: 71°
Forth Contact (external): 11:23:53 UT, 13:23:53 Local Time. Sun Elevation: 71°
Photo Camera CANON EOS 300D (Digital)
Tele EF 300 mm f/4 LIS
CANON Extender EF 2X
Remote Switch RS -60E3
Tripod and micrometric head
Solar filters during partial phases: aluminized mylar (white) metallic-coated glass (red).
Each photo is associated with the time of the shot provided by Canon digital camera but corrected by a clock synchronized with the Frankfurt time station. The photos are taken shortly after the local time 10:00 AM when the planet Venus had already traveled more then half of its transit across the sun. The photos are presented in pairs taken close in time, the first with the metallic-coated glass filter that gives a red image and the second with the aluminized mylar filter that gives a white image. Between a pair and the other spend about 15 to 30 minutes. After the local time 13:00 AM, to the proximity of the point of the second contact (internal), it is preferred and take more pictures in succession with the aluminized mylar filter (white). The angular diameter of the apparent Sun and 31.5' (about half degree), that of Venus of 58" (seconds). The apparent path of Venus within the solar disc is approximately a straight line if you look at the phenomenon through a telescope with equatorial mount. Instead using horizontal reference system, as in the present case, the path is a curve because the celestial pole rotates with respect the local zenith during transit duration and the last part the path bends in the direction of the edge.
PHOTOS DURING THE TRANSIT
Shot local time: 10h 13m 18s,
Sensitivity: 100 ISO, Exposure: 1/320,
Aperture: 16
Solar filter: mmetallic-coated glass.
Solar folter: in mylar.
Solar filter: metallic-coated filter.
Solar filter: in mylar.
Solar filter: metallic-coated glass.
Solatr Filter: in mylar.
Sensibilità: 100 ISO, Exposure: 1/400,
:Solar filter: in mylar.
Filtro solare: in mylar.
Sensibilità: 100 ISO, Posa: 1/400,
Diaframma: 16
PHOTO REMARKS
The first two pairs of photos were taken around the maximum of the transit, the other four pairs every 30 minutes but the last were delayed because of clouds. The last two pairs, after 13:00, around to the forth (external) contact of Venus, at about 1:17 PM, were also disturbed by clouds which has not allowed to follow the transit near the Sun edge and observe the phenomenon of "black-drop".
Assuming that Venus maintains a constant speed in the last phase of the path through the Sun, this can be calculated from the penultimate pair of photos as the ratio between the distance travelled by apparent diameter of Venus Sv, and the time between the two photos (T2-T1) which is 11,25 minutes.
By the last two photos you can determine the displacement Sx of Venus necessary to reach the third contact (inner) in a time tx with a speed Sx/tx = Sv/(t2-t1) calculated previously. from this equation we derive the time tx = (t2-t1)*Sx/Sv. The ratio between the two distances Sx/Sv, measured by enlarging the photos, proves to be about 0,33 and Tx = 0.33 11.25 = 3.71 minutes.
The time of the third internal contact is T1 + tx = 13h 16s + 3.71 m = 13h 3.98 m = 13h 3m 59s.
The result is only 34 seconds less than the theoretical value of 13h 4m 33s. Since the error on times is less than 0.5 seconds, all the error is concentrated on the distances, difficult to measure with an accuracy better than 10 %. The maximum error in the difference between two distances would be thus 20% while it is an error of about 15% (the ratio of the distances should be 0.38 instead of 0.33).
Alberto Aiosa is member of amateur astronomers group Hipparcos of Rome, Italy.
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