Black drop

Black Drop is a phenomenon observed during the transit of Venus and Mercury across the Sun. The effect appears as a small, black teardrop shape that connects the planet and the Sun's edge as the planet starts to ingress or egress. The black drop effect has been a significant factor in historical attempts to measure the Astronomical Unit.

History

The black drop effect was first reported during the 1761 transit of Venus. Early observers, attempting to time the exact moment of contact, noticed that as Venus began to ingress, a small black "drop" seemed to stretch from the Sun's edge to the planet. This made it difficult to time the exact moment of contact accurately, leading to discrepancies in measurements of the Astronomical Unit.

Causes

The exact cause of the black drop effect has been a subject of debate among astronomers. Initially, it was attributed to Venus's thick atmosphere. However, the effect is also observed during the transit of Mercury, which has a much thinner atmosphere, suggesting that other factors are involved.

Modern research suggests that the black drop effect is primarily caused by two factors: the point spread function of the observing telescope, and the limb darkening effect, where the edge of the Sun appears darker due to the scattering of light in the Sun's atmosphere.

Observations

The black drop effect has been observed during several transits of Venus and Mercury. The effect is most pronounced when observed through a telescope, but can also be seen in high-resolution photographs and digital images.

Impact on Astronomical Measurements

The black drop effect has historically had a significant impact on attempts to measure the Astronomical Unit. The timing of the exact moment of contact during a transit is crucial for these measurements. The black drop effect makes this timing difficult to determine accurately, leading to discrepancies in the calculated value of the Astronomical Unit.

See Also

• Transit of Venus
• Transit of Mercury
• Astronomical Unit
• Limb darkening