Constellation of the Month: Reticulum, the Reticle
Just sticking up from our southern horizon is the northern panhandle of the constellation Reticulum, the Reticle. The majority of the constellation is below our horizon, including the four brightest stars that form the shape of a diamond.
Click the image for a larger map
Though faint, the diamond asterism is rather recognizable, and in geometry it is called a rhombus. This constellation was first named the Rhombus by Isaac Habrecht in 1621. Habrecht was a clockmaker who, along with his brother Josias, was hired to build the second astronomical clock between 1571 and 1574. Jakob Bartsch, a German astronomer, published a series of star charts in 1624 in which this little constellation was renamed Reticulum by the French astronomer Nicolas Louis de Lacaille, who also named a number of other southern constellations.
In his Memoirs published by the French Royal Academy of Sciences in 1756, Lacaille recounted his reasoning in naming this constellation after "the little instrument used to construct this catalogue. It is constructed by the intersection of four lines drawn from each corner of a square to the middle of the two opposite sides."
The reticle in a telescope is a glass disc at the mutual focal point of the telescope's main lens or mirror and eyepiece. The glass disc has lines scribed on it, with these lines often in the form of a crosshair. Sometimes other lines are scribed on the glass as well. These are used for various astronomical purposes. One use is to measure the angular distance and position angle between the members of multiple-star systems. Astronomers also use a similar device called a filar micrometer to make position angle and distance measurements between the components of a binary star.
Astronomers use position angle and distance to measure the location of one object relative to another on the sky. For double stars, the position of the secondary in the double star is measured relative to the primary. When the secondary star is directly north of the primary star, its position angle is zero degrees. When the secondary is directly east of the primary, that's 90 degrees, and so on: directly south, 180 degrees, and directly west, 270 degrees. With the position angle and distance in arc-seconds, the secondary star's position can be uniquely described relative to the primary.
An interesting double star in Reticulum is Zeta Reticuli, a binary star whose two components are magnitude 5.2 and 5.5. Zeta2 Reticuli (the secondary) is 309 seconds-of-arc from Zeta1 Reticuli (the primary). Zeta2 is north and a little east of Zeta1, at a position angle of 35.7 degrees. These two stars are similar, but slightly smaller than our Sun. They are located just over 39 light-years from the Earth.
These two stars are both naked eye and easily resolved, making them one of the few real naked-eye binary stars in the sky. Most of the other visual double stars are actually two stars that are both along the same line of sight, but in fact unrelated. The Zeta Reticuli system stars orbit around each other in about 170,000 years. Not only is it rare for this pair of stars to be a visual binary, but for them both to be sun-like stars is unique.
As if this were not enough, this star is famous in the annals of UFOs. In the 1960s, Barney and Betty Hill could not remember what had happened to them one night as they were driving home. After post-hypnotic suggestion, they remembered being abducted by aliens (gray-colored creatures that have been nicknamed "The Grays"). The Hills remembered seeing a star map in the alien spacecraft. Betty was able to draw the map later and astronomers determined the home world of the Grays orbited around Zeta Reticuli. Unfortunately, observations meant to discover large planets around these stars have not uncovered any to date. If NASA can get its Terrestrial Planet Finder (TPF) spacecraft built and launched, we may be able to find the Grays' home, if it exists, since this pair are prime targets for this mission.
The Planets for January 2011
Jupiter is high in the west as it gets dark on these January nights. Drifting slowly eastward in Pisces, Jupiter's disc is 37.1 seconds-of-arc across at midmonth and sets around 10:30 p.m. The King of the Gods is magnitude -2.3 at midmonth. Jupiter's South Equatorial Belt, which faded away in late 2009, has returned, getting darker and darker as time goes on.
Saturn is also moving slowly eastward, but it is in central Virgo this month. Rising around 11:45 p.m., the Ringed Planet's disc is 17.6 seconds-of-arc across. The Rings are tipped down 10.3 degrees with the northern face showing and span 39.9 seconds-of-arc across.
Around 3:45 a.m., Venus comes over the eastern horizon at magnitude -4.4. Its disc is 23.0 seconds-of-arc across and 54% illuminated at midmonth. Venus starts the month as a crescent in the constellation Libra. It slices eastward across the very northern panhandle of Scorpius and then into Ophiuchus, becoming less of a crescent each day
Mercury pops out from the bright solar glare into the morning sky at the end of December. As January starts, Mercury will be moving slowly south in Ophiuchus, and then turning eastward into Sagittarius. It will pass over the Teapot asterism just after midmonth. At the beginning of the month, Mercury will be 56% illuminated, shining at magnitude -0.2. Midmonth will see Mercury's disc at 6.0 seconds-of-arc across and shrinking. By the end of the month, Mercury will be magnitude -0.3 and 90% illuminated as it drops back into the solar glare.
Earth passes its closest point to the Sun at noon on Jan. 3. If you are looking for a meteor shower to observe, the Quadrantids will peak around the same time. Bundle up to stay warm and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.