Pedro Castle, Friday 22nd June, 2012

The viewing of the Transit of Venus was, if you weren’t there, a huge success. Fortunately, we had seven telescopes for more than a thousand people to look through, and the atmosphere was orderly and convivial.

    Early risers can see Venus and Jupiter before and in the dawn, while Saturn, Mars and Mercury are visible when darkness falls.

      Saturn is close to the 15^th brightest star in the sky, Spica, which lies in the constellation of Virgo. Roughly halfway between this brilliant duo and the star Regulus in Leo, Mars glows distinctly in an area of dim stars. Mercury is visible as the sun sets, a good excuse to go the beach for the waning of the day.

    Further north from this line of the ecliptic, the bright star Arcturus is unmistakable. In the illustration above it is the alpha, α, orange star in the kilt of the Herdsman, Boötes.

    In ancient Babylon the stars of Boötes were known as SHU.PA. They were apparently depicted as the god Enlil, who was the leader of the Babylonian pantheon and special patron of farmers.

    Exactly whom Boötes is supposed to represent in Greek mythology is not clear. According to one version, he was a ploughman who drove the oxen in the constellation Ursa Major using his two dogs Chara and Asterion (from the constellation Canes Venatici). The oxen were tied to the polar axis and so the action of Boötes kept the heavens in constant rotation.

     The Boötes void, or the Great Void is a huge and approximately spherically shaped region of space, containing very few galaxies. It is located in the vicinity of the constellation Boötes, hence its name. At nearly 250 million light-years in diameter (approximately 0.27% of the diameter of the visible universe), the Boötes void is one of the largest known voids in the universe, and is referred to as a supervoid. According to astronomer Greg Aldering, the scale of the void is such that "If the Milky Way had been in the center of the Boötes void, we wouldn't have known there were other galaxies until the 1960s."

   The Boötes Dwarf Galaxy is a faint, satellite galaxy of the Milky Way located in Boötes about 197 000 light-years  away from Earth.

     To the naked eye, orange-yellow Arcturus has a visual magnitude of −0.04, making it the brightest star north of the celestial equator, and the fourth brightest star in the night sky, after −1.46 magnitude Sirius, −0.86 magnitude Canopus, and −0.27 magnitude Alpha Centauri. However, Alpha Centauri is a bright binary star, whose unresolved components to the naked eye are both fainter than Arcturus. This makes Arcturus the third brightest individual star, just ahead of Alpha Centauri A (α Cen A), whose visual magnitude is −0.01.

     The apparent (visual) magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude.