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Astronomical unit (AU)

Astronomical unit is used to express distances in the solar system. The astronomical unit is a conventional unit of length equal to 149 597 870 700 m exactly.

Some examples:

  • The Earth is 1.00 ± 0.02 AU from the Sun
  • The Moon is 0.0026 ± 0.0001 AU from the Earth
  • Mercury is 0.39 ± 0.09 AU from the Sun
  • Venus 0.72 ± 0.01 AU
  • Mars 1.52 ± 0.14 AU
  • Jupiter 5.20 ± 0.25 AU

Aureole effect

During transits of Venus, a bright arc has often been reported, seen around the edge of the Venus disk which is partially outside the solar limb. This is called the aureole effect and it was first described by Russian scientist Mikhail Lomonosov when he observed the transit of Venus in 1761. This peculiar aspect of the planet has been observed in all subsequent transits, with varying intensity and size.

As promptly inferred by the early observers, the aureole has to be ascribed to the presence of an atmosphere around Venus. In some sense, it can be considered as the first solid confirmation of its presence! The arc of light is caused by refraction of sun light in the dense upper atmosphere of Venus.


The mysterious Black Drop Effect

The black drop effect is the apparent smearing of the edge of Venus against the edge of the sun during a transit of Venus.  The ligament joining the two objects makes it harder to time the exact second when the edges appear to make contact, thus diminishing the value of using a transit of Venus to measure the size of the solar system as Edmond Halley and others had proposed

While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury, which was  observed by the TRACE spacecraft.  The cause of the black drop effect is limb darkening and point-spread function as explained on this page.

Ciel et Terre - Bruxelles, Revue populaire d'Astronomie et
de Météorologie, 3ème année, 1882 - page 412-41

The phenomenon was first described by Lalande in an article for the Academy of Sciences (in French) in 1769.

You can see a similar anomaly if you almost pinch your thumb and forefinger together.  Just before you sense contact, a black feature spans your two digits.



During the transit of a planet in front of its star occur what are called "contacts". They occur when the circumference of the small object seems to touch the circumference of the large object at a single point. There are 4 contacts happening in a definite order, as shown below.



A coronagraph is a telescopic attachment designed to block out the direct light from a star so that the corona of the star, as well as nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. This technique allows the detection of objects with a brightness millions or even billions of times smaller than that of the object blocked out.

Most coronagraphs are intended to view the corona of the Sun, but since the 80s, a new class of conceptually similar instruments (called stellar coronagraphs to distinguish them from solar coronagraphs) are being used to find extrasolar planets, i.e. exoplanets, around nearby stars.



An exoplanet, or extrasolar planet, is a planet orbiting a star other than the Sun. 763 are listed to date (10/04/2012).




A helioscope is an instrument used in observing the sun. The method involves projecting an image of the sun onto a white sheet of paper suspended in a darkened room with the use of a telescope. This instrument was invented during the 17th century.


The first heliotropii telioscopici or helioscope created by C. Scheiner (1575-1650) to assist his sunspot observations (ca. 1638)


James Clerk Maxwell Telescope

The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum (far-infrared to microwave). Scientists use it to study our Solar System, interstellar dust and gas, and distant galaxies.

The JCMT is funded by a partnership between the United Kingdom, Canada, and the Netherlands. It is operated by the Joint Astronomy Centre and was named in honour of mathematical physicist James Clerk Maxwell.



In astronomy, limb refers to the edge of the disk of a celestial body; the lunar limb is thus the edge of the disk of the moon.


Mauna Kea Observatories

The Mauna Kea Observatories are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawai'i, USA. The facilities are located in a 2.0 km² special land use zone known as the "Astronomy Precinct", which is located in the Mauna Kea Science Reserve.

The location is ideal because of its dark skies, good astronomical seeing, low humidity and position above most of the water vapor in the atmosphere, clean air, good weather and almost equatorial location.



Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight. Knowing the parallax of an object is to know its distance.parallaxe

One way to measure the distance between the Earth and a planet in the solar system is by measuring the angle to the planet when viewed from the Earth. The half-value of this angle is called the parallax.

In practice, the measurement is made from two observation points, A and B, located on Earth. The measure of angle A-Planet-B enables determination of the angle Centre of the Earth-Planet-A (or B) called diurnal parallax.

The solar parallax is 8.794 148", which means that from the Sun, we would see the radius of the Earth (at its equator) as occupying an angle of 8.794 148"

Source: G. Javaux (in french)



refractionRefraction is the bending of light rays passing from one transparent medium to another.

Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude.


Space Science Institute

The Space Science Institute (SSI) in Boulder, Colorado, is a nonprofit, public benefit corporation formed in 1992. Its purpose is to create and maintain an environment where scientific research and education programs can flourish in an integrated fashion.

SSI's research program encompasses the following areas: space physics, earth science, planetary science, and astrophysics.


Transit method

The transit method is an indirect method to detect exoplanets. When a planet passes in front of its star it causes a very slight decrease in the amount of light received from the star, as can be seen in this image showing the transit of Venus in 2004.

telluric planet
Courtesy of D. Ehrenreich - "Venus as a transiting exoplanet" - 3rd Europlanet strategic workshop - 4th PHC/Sakura meeting - 5-7 March 2012, Paris.

The transit method therefore primarily consists of repeated observations of a maximum number of stars, for long periods of time (years). The efficiency of this method depends mainly on the number of observations, their accuracy and the (unknown) fraction of planets that have the correct orbital inclination around their star to give them a transiting orbit when viewed from the Earth, as well as a distance to the star that makes transits frequent enough to give us a chance of detecting them.

If one transit is detected, the star is then closely monitored to confirm the transit. A true transit will be repeated periodically. If this is the case then studying the information observed during transits can give access to lots of information: the mass of the planet, its radius - and thus its density, its distance from the host star, etc.


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