Black drop effect
Encyclopedia
Transit of Venus
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun...
and, to a lesser extent, a transit of Mercury
Transit of Mercury
A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....
.
Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
' disk to the limb of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, making it impossible to accurately time the exact moment of second or third contact. This led to the failure of the attempts during the 18th century transits of Venus to establish a truly precise value for the astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
.
The black drop effect was long thought to be due to Venus' thick atmosphere, and indeed it was held to be the first real evidence that Venus had an atmosphere. However, it is now thought by many to be an optical effect caused by the smearing of the image of Venus by turbulence in the Earth's atmosphere or imperfections in the viewing apparatus. With precise measurements a black drop effect was observed from outside the Earth's atmosphere during the 1999 and 2003 transits of Mercury
Transit of Mercury
A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....
, although Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
has no significant atmosphere.
In the June 8, 2004 transit of Venus, many observers reported that they did not see the black drop effect, or at least that it was much less pronounced than had been reported in earlier centuries' transits. Larger telescopes and better optics may have been a factor.
External links
- Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus—abstract at AASAmerican Astronomical SocietyThe American Astronomical Society is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC...
website - The black drop effect
- The "Black Drop" effect—explanation at the TransitOfVenus.org website
- Sky and Telescope: Where Was the Black Drop?