ESA’s missions gear up for the transit of Venus
Here’s a quick-look summary of what we can expect from our space-based missions during the transit:
Venus Express: solar occultation experiment – using the Sun’s light to analyse Venus’ atmosphere, an important technique for exoplanet studies.
Proba-2: Watching for the dip in solar brightness as soon as Venus’ thick atmosphere makes contact with the Sun’s disc.
Hinode: visible, X-ray and UV observations to study the black drop effect and aureole.
Venus approaching the Sun, seen by SOHO on 4 June. Credits: SOHO/ESA/NASA
SOHO: views of Venus as it approaches and leaves the Sun.
Hubble: using the Moon to collect diffuse reflected sunlight, which will contain a tiny portion of the light that passed through Venus’ atmosphere.
For a more in-depth review, see our news story ESA missions gear up for the transit of Venus, published on the space science web portal yesterday.
Measuring the size of the Solar System – the ‘black drop’ problem
By guest blogger Peter Bond
Despite the best efforts by astronomers who voyaged to far flung reaches of the Earth to watch the transits, the results of the observations was not as conclusive or accurate as had been hoped. The observations were plagued by many technical difficulties, and by the slightly fuzzy outline of Venus, caused by its dense atmosphere. There was also an unforeseen problem with a phenomenon known as the ‘black drop’ effect.
One of the chief problems the observers faced was pinpointing the precise time of ‘second contact’, when the whole of Venus was first visible on the face of the Sun. They noticed that its black disc seemed to remain linked to the edge of the Sun for a short time by a dark ‘neck’, making it appear almost pear-shaped. The same happened in reverse when Venus began to leave the Sun.
Click for an animation of the black drop effect.
This so-called ‘black drop effect’ was one of the main reasons why timing the transits failed to produce consistent accurate results for the Sun-Earth distance. Halley expected second contact could be timed to within about a second. The black drop reduced the accuracy of timing to more like a minute.
The black drop effect is often mistakenly attributed to Venus's atmosphere, but modern research has suggested that it is due to a combination of two key effects. One is the image blurring that takes place when a telescope is used (described technically as ‘the point spread function’). The other is the way that the brightness of the Sun diminishes close to its visible ‘edge’ (known to astronomers as ‘limb darkening’). There may also be a small contribution from observing through Earth’s atmosphere, but observations of the black drop effect during Mercury’s 1999 transit across the Sun using NASA’s TRACE satellite confirmed that neither the planet’s nor Earth’s atmosphere is needed to produce the effect.
Despite the disappointments of the 18th century expeditions, optimistic astronomers tried again during the transits of 9 December 1874, and 6 December 1882. Once again the results were inconclusive, and scientists began to realise that the practical problems with Halley’s method were just too great to overcome. Nevertheless, the value of the Sun-Earth distance was known with much greater accuracy than ever before after the results of the 1882 transit were analysed.
Read more about Halley's method in part 3: Parallax
Key phases during a transit of a planet across the face of the Sun are often referred to as 1st, 2nd, 3rd and 4th contact. Credit: Michael Zeiler, eclipse-maps.com
Astronomers use different terms to describe the four main phases of a transit:
- Ingress, exterior (or first contact): the point at which Venus’ disc is just touching the outer edge of the Sun. Shortly after, the planet appears to make a small black indent on the solar disc.
- Ingress, interior (or second contact): the point at which the entire planet has moved onto the solar disc.
- Egress, interior (or third contact): the point at which the planet touches the opposite solar limb.
- Egress, exterior (or fourth contact): the point at which Venus is just outside the Sun’s disc, concluding the transit.
Here are some other useful transit terms:
Image showing the aureole observed during the 2004 egress of Venus with the Dutch Open Telescope in La Palma - Credit: Tanga et al. 2012
Aureole: the bright arc seen around the circumference of Venus’ disc partially outside the solar limb during ingress and egress. It was first observed during the transit of 1761 and revealed that Venus has an atmosphere. The effect is caused by refraction of sunlight in the dense upper atmosphere of Venus.
Black drop effect: the small black teardrop shape that appears to connect Venus to the limb of the Sun as it fully enters the solar disc just after ingress, interior, and just before egress, interior as it begins to leave. It is thought to be an optical effect caused in part by the effect of observing through Earth’s atmosphere, combined with diffraction of light inside the telescope, and by the dimming of the intensity of the Sun’s surface just inside its apparent outer edge.
Greatest transit: the point at which Venus is in the middle of its path across the solar disc, marking the halfway point in the timing of the transit.