The above image is a series of drawings of the black-drop effect as depicted by Swedish chemist and mineralogist Torbern Olaf Bergman (1735 - 1784). Bergman made the drawings based on his observations of the 1761 transit of Venus. Image Credit: The image is in the Public Domain.
We have seen that the 1761 transit of Venus was historic for several reasons. We now consider another: the first recording of the black-drop effect.
Before we go too far, we should review some terminology. The timing of a transit depends on the recording of four critical contact points. These points are quite logically labeled "first," "second," "third," and "fourth contact." First contact is the point when the transiting body first appears to touch (or bites into) the disc of the sun. Second contact is the point when the transiting body completely moves into the disc of the sun. Third contact is the point when the transiting body breaks the far edge of the sun's disc. And fourth contact is the point when the transiting body completely clears the sun's disc.
British astronomer Edmund Halley had encouraged international cooperation in observing the 1761 transit of Venus and had stressed the importance of accurately recording — to the second, the thought — the times of these four points of contact. The data from the various expeditions could then be use to determine the solar parallax and, from that, the distance from Earth to the sun — a foundational measurement in astronomy that has come to be known as the Astronomical Unit.
But the observing teams ran into difficulties in recording the times for second contact and third contact. It seemed there was not a clear agreement on the point of completely moving onto the disc (for second contact), or the point of touching the the edge of the disc (for third contact). In fact, within the same teams, different observers recorded different times.
This optical phenomenon has come to be known as the "black-drop effect." The effect gets its name from the shape created as the transiting body moves onto and off of the sun's disc. At the points of second contact and third contact, the transiting body and the nearest edge of the sun's disc appear to elongate toward each other, thus creating a shape that resembled a drop of black fluid. Some refer to this shape as a "tear drop."
Over time, different causes have been suggested for the phenomenon. At first, it was suggested that the phenomenon was evidence that Venus had an atmosphere. Later, it was suggested that the phenomenon was caused by turbulence in Earth's atmosphere or by imperfections in the viewing aparatus.
While it was later confirmed by other methods that Venus had an atmosphere, this was eventually ruled out as the reason for the black-drop effect, because the phenomenon has also be seen during transits of Mercury, which has no significant atmosphere. As to the cause being atmospheric turbulence, this possibility was eliminated following the 1999 and 2003 observations of the transit of Mercury, which were made from Earth orbit using NASA's Transition Region and Coronal Explorer (TRACE). During both transits the black drop effect was observed.
So what is the cause of the black-drop effect? An answer is proposed by Jay M. Pasachoff, Glenn Schneider, and Leon Golub, the ones who studied the results of the 1999 and 2003 Mercury transits observed by TRACE. According to the trio, the cause of the black-drop effect is a combination of two factors: the point-spread function of the imaging system and solar limb darkening.
A point-spread function (PSF) is the method by which an imaging system responds to a point source or point object. The imaging system may be artificial, as with a CCD devices, and it may be natural, as with human eyes.
Solar limb darkening refers to the gradual diminishing of the sun's brightness as one moves from the center to the edge of the solar disc. This darkening is in turn the result of two effects. The first is that the density of the star diminishes as the distance from the center increases. The second is that the temperature of the star diminishes as the distance from the center increases.
The team found that when the these two factors were removed, the black-drop effect was not evident. The team's final report is available to read online: "Space Studies of the Black-Drop Effect at a Mercury Transit." G. Schneider, J. M. Pasachoff, L. Golub. Submitted October 14, 2003. Cornell University Library. URL: arxiv.org/abs/astro-ph/0310379v1
The next transit of Venus will occur over June 5-6. To learn more, visit these links.
2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html
Transit of Venus, Sun-Earth Day 2012, NASA: sunearthday.nasa.gov/transitofvenus/
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