Pedro Castle, June 25th. 7.30 p.m. 2009

Close to the zenith and easily identifiable is the bright star Spica, which is the alpha star in the constellation Virgo.
Virgo is one of the constellations of the zodiac. Its name is Latin for virgin,). Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky.
Because of the presence of a galaxy cluster (consequently called the Virgo cluster) within its borders), this constellation is especially rich in galaxies.
Illustrated above, some examples are Messier 49 (elliptical), Messier 58 (spiral), Messier 59 (elliptical), Messier 60 (elliptical), Messier 61 (spiral), Messier 84 (lenticular), Messier 86 (lenticular), Messier 87 (elliptical and a famous radio source), Messier 89 (elliptical) and Messier 90 (spiral).
A noted galaxy that is not part of the cluster is the Sombrero Galaxy (M104), an unusual spiral galaxy. It is located about 10° due west of Spica.
Spica (also known as α Vir / α Virginis / Alpha Virginis) is the 15th brightest star in the nighttime sky. It is 260 light years distant from Earth. A blue giant, it is a variable of the Beta Cephei type.

Who exactly Virgo was considered to represent is uncertain; in history, it has been associated with nearly every prominent goddess, including Ishtar, Isis, Cybele, and Athena, as well as with Mary the Mother of Jesus.
Persephone (who in some mythologies, notably the Eleusinian Mysteries, was considered to be a form of Demeter) is often mentioned as well, Virgo being visible mainly in the spring months when she was believed to have risen from the underworld.
According to one interpretation, the constellation depicts Astraea, the virgin daughter of the god Zeus and the goddess Themis. Astraea was known as the goddess of justice, and was identified as this constellation due to the presence of the scales of justice Libra nearby, and supposedly ruled the world at one point with her wise ways until mankind became so callous she returned to the skies disgusted.
Saturn is visible as darkness falls, but for the other planets you will either stay up late, to see Jupiter, which rises after 10 pm, or get up before dawn to see Venus and Mars.

Pedro Castle,May 27th, 2009, 7.30 p.m.

In the rainy season the decision whether or not to view is determined by the amount of cloud cover. If more than 50% of the sky is obscured, we cancel. This can sometimes be a difficult judgment call.
The constellation illustrated above is Leo, the Lion, which is visible overhead as darkness falls. The sickle shape, or backwards question mark with the bright star Regulus at its base, is easy to discern.
Brighter Saturn is below the question mark.
Regulus, which means the heart of the lion, is a hot, blue star which has a luminosity about 140 times that of the Sun and about 3.4 times the diameter. The star is in fact the A component of a multiple star system. The B and C components are a close binary pair revolving together about 100 AU (Astronomical Unit) from the A star. This is two and a half time the distance of Pluto and the Sun.
The Sumerians are recorded as seeing this group of stars as a lion. The form of the Lion was probably passed along to the Babylonians, Greeks, Roman and others. The Egyptians worshipped the lion because the sun was in this constellation at the time of the life-giving floods of the Nile.
In Greek mythology, it was identified as the Nemean Lion which was killed by Herakles during one of his twelve labours, and subsequently put into the sky by the goddess Hera, the vengeful wife of his father, Zeus.

Pedro Castle,April 28th, 7.00 p.m.

The zenith at 7 on Tuesday will be just across the boundary line separating Gemini from Cancer, the constellation illustrated above.
Cancer, the Crab, plays a minor role in the Twelve Labors of Hercules. While Hercules was busy fighting the multi-headed monster, Hydra, the goddess Hera, who did not like Hercules, sent the Crab to distract him. Cancer grabbed onto the hero's toe with its claws, but barely breaking the rhythm of his great battle with Hydra, Hercules crushed the crab with his foot. Hera, grateful for the little crustacean's heroic but pitiful effort, gave it a place in the sky.

Cancer is famous despite itself due to its Zodiacal origins. In fact, it is very faint with no stars brighter than 4th magnitude. However, it is home to Praesepe (M44), an open cluster also called the Beehive Cluster or the Gate of Men. In the picture above, it is the star marked next to the lower eyestalk. It is one of the nearest open clusters to the Solar System, and it contains a larger star population than most other nearby clusters. Under dark skies the Beehive Cluster looks like a nebulous object to the naked eye; thus it has been known since ancient times.

Like many star clusters of all kinds, Praesepe has experienced mass segregation. This means that bright, massive stars are concentrated in the cluster's core, while dimmer, less massive stars populate its halo (sometimes called the "corona"). The cluster's core radius is estimated at 3.5 parsecs (11.4 light years); its half-mass radius is about 3.9 parsecs (12.7 light years); and its tidal radius is about 12 parsecs (39 light years). However, the tidal radius also includes many stars that are merely "passing through" and not bona fide cluster members.
Altogether, the Praesepe cluster contains at least 1000 gravitationally bound stars, for a total mass of about 500-600 Solar masses. A recent survey counts 1010 high-probability members, of which 68% are M dwarfs, 30% are Sun-like stars of spectral classes F, G, and K, and about 2% are bright stars of spectral class A. Also present are five giant stars, four of which have spectral class K0 III and the fifth G0 III.
So far, eleven white dwarfs have been identified, representing the final evolutionary phase of the cluster's most massive stars, which originally belonged to spectral type B. Brown dwarfs, however, are extremely rare in this cluster, probably because they have been lost by tidal stripping from the halo.
Galileo was one of the first to study this open cluster with optical aid and counted forty stars.
Praesepe is Latin meaning crib or manger. Some ancient lore is associated with it: Greeks and Romans saw this "nebula" as the manger (Greek: Phatne) associated with two asses who eat from it, Asellus Borealis, the Northern Ass and Asellus Australis, the Southern Ass. Erathosthenes reported that these were the asses on which the gods Dionysos and Silenus rode into the battle against the Titans, who were frightened by the animals' braying so that the gods won. As a reward, the asses were put in sky together with Phatne.
The wine must have been good in ancient Greece.
Aratos (260 B.C.) mentioned this object as "Little Mist", Hipparchus (130 B.C.) included this object in his star catalog and called it "Little Cloud" or "Cloudy Star." Ptolemy, the Roman[1] mathematician, astronomer, geographer and astrologer mentions it as one of seven "nebulae" he noted in his Almagest.
According to the new determination by ESA's astrometric satellite Hipparcos, the cluster is 577 light years distant (previous estimates have been at 522 light years), and its age was estimated at about 730 million years.
Open clusters are physically related groups of stars held together by mutual gravitational attraction. Therefore, they populate a limited region of space, typically much smaller than their distance from us, so that they are all roughly at the same distance. They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.
Saturn is in the constellation Leo, shining in bright competition with Regulus.

Pedro Castle, March 29th, 7.00 p.m.

The Pleiades, known as the Seven Sisters, will be close to the crescent Moon on the 29th March . This is a nearby, fairly loose open cluster of about 500 stars in the constellation Taurus. Although it is also known as the Seven Sisters, most people can see only six stars, all of them bright blue-white B or Be stars: Alcyone, Atlas, Electra, Maia, Merope, and Taygeta (Pleione is the seventh brightest).
Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.
They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.
The distance to the Pleiades is an important first step in the so-called cosmic distance ladder, a sequence of distance scales for the whole universe. The size of this first step calibrates the whole ladder, and the scale of this first step has been estimated by many methods. As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.
The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione.
The Pleiades are a prominent sight in winter in the Northern Hemisphere and in summer in the Southern Hemisphere, and have been known since antiquity to cultures all around the world, including the Māori and Australian Aborigines, the Persians (who called them Parveen/parvin), the Chinese, the Maya (who called them Tzab-ek), the Aztec (Tianquiztli), and the Sioux of North America. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey. They are also mentioned three times in the Bible (Job 9:9 and 38:31, as well as Amos 5:8). The Pleiades (Krittika) are particularly revered in Hindu mythology as the six mothers of the war god Skanda, who developed six faces, one for each of them. Some scholars of Islam suggested that the Pleiades (At-thuraiya) are the Star in Najm which is mentioned in the Quran.
Saturn is risen in the east (magnitude +0.5, near the hind foot of Leo) and is just past opposition. The planet is well up in the southeast by midevening, and highest in the south around midnight. Don't confuse Saturn with slightly fainter Regulus 18° (nearly two fist-widths at arm's length) to its upper right in early evening, and more directly to Saturn's right late at night.
Saturn's rings are 3½° from edge on. The rings will open to 4° by late May, then will close to exactly edge-on next September 4th — when, unfortunately, Saturn will be out of sight practically in conjunction with the Sun.

March 2nd 2009

The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. This is caused by the different angles from which we see the bright part of the Moon's surface. These are called "phases" of the Moon. Of course, the Moon doesn't generate any light itself; it just reflects the light of the Sun. The Moon passes through four major shapes during a cycle that repeats itself every 29.5 days. The phases always follow one another in the same order.
Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages, called phases. The phases designate both the degree to which the Moon is illuminated and the geometric appearance of the illuminated part. These phases of the Moon, in the sequence of their occurrence (starting from New Moon), are listed below.
(1) New Moon - When the Moon is roughly in the same direction as the Sun, its illuminated half is facing away from the Earth, and therefore the part that faces us is all dark: we have the new moon. When in this phase, the Moon and the Sun rise and set at about the same time. (2) Waxing Crescent Moon - As the Moon moves around the Earth, we get to see more and more of the illuminated half, and we say the Moon is waxing. At first we get a sliver of it, which grows as days go by. This phase is called the crescent moon. (3) Quarter Moon - A week after the new moon, when the Moon has completed about a quarter of its turn around the Earth, we can see half of the illuminated part; that is, a quarter of the Moon. This is the first quarter phase. (4) Waxing Gibbous Moon - During the next week, we keep seeing more and more of the illuminated part of the Moon, and it is now called waxing gibbous (gibbous means "humped"). (5) Full Moon - Two weeks after the new moon, the moon is now halfway through its revolution, and now the illuminated half coincides with the one facing the Earth, so that we can see a full disk: we have a full moon. As mentioned above, at this time the Moon rises at the time the Sun sets, and it sets when the Sun rises. If the Moon happens to align exactly with the Earth and Sun, then we get a lunar eclipse. (6) Waning Gibbous Moon - From now on, until it becomes new again, the illuminated part of the Moon that we can see decreases, and we say it's waning. The first week after full, it is called waning gibbous. (7) Last Quarter Moon - Three weeks after new, we again can see half of the illuminated part. This is usually called last quarter. (8) Waning Crescent Moon - Finally, during the fourth week, the Moon is reduced to a thin sliver from us, sometimes called waning crescent. A while after four weeks (29.5 days, more precisely) the illuminated half of the Moon again faces away from us, and we come back to the beginning of the cycle: a new moon. Sometimes, when the Moon is almost new, it is possible to dimly see its darkened disk. The light from the Sun cannot reach this part of the Moon directly; but at this time the Earth (as viewed from the Moon) is at its full and very bright, and what we see is light reflected from the Earth, that then bounces back at us from the Moon. It's a long trip for this light: from the Sun to the Earth, to the Moon, and back to the Earth, about 480,000 miles in total.
Venus (visible in the west) is the next closest visible heavenly body, but it is never closer than 25,000,000 miles from the earth. The moon appears to be as large as it is because of its closeness, but it is comparatively small since it is only a quarter of the diameter of the earth.
The sun is a hundred times the diameter of the earth, but it is also about 93,000,000 miles away. It just happens that size and distance cancel each other and both the sun and the moon appear to be the same size. Although the moon is moving from west to east, the earth's spin overtakes it and makes it seem to move from east to west. It appears to "rise" in the east, moves westward, and "sets" in the west.

January 2009, Pedro Castle 7.00 pm

The extremely bright object high in the western sky is the planet Venus.
Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days.
The planet is named after Venus, the Roman goddess of love. It is the brightest natural object in the night sky, except for the Moon, reaching an apparent magnitude of −4.6. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the Evening Star.
Classified as a terrestrial planet, it is sometimes called Earth's "sister planet," because they are similar in size, gravity, and bulk composition. Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light; this was a subject of great speculation until some of its secrets were revealed by planetary science in the twentieth century. Venus has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide, as it has no carbon cycle to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. It has become so hot that the earth-like oceans that the young Venus is believed to have possessed have totally evaporated, leaving a dusty dry desertscape with many slab-like rocks. The best hypothesis is that the evaporated water has dissociated, and with the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind. The atmospheric pressure at the planet's surface is 92 times that of the Earth.
Venus rotates once every 243 days—by far the slowest rotation period of any of the major planets. To an observer on the surface of Venus the time from one sunrise to the next would be 116.75 days.
As the brightest point-like object in the sky, Venus is a commonly misreported 'unidentified flying object'. U.S. President Jimmy Carter reported having seen a UFO in 1969, which later analysis suggested was probably the planet, and countless other people have mistaken Venus for something more exotic.
As it moves around its orbit, Venus displays phases in a telescopic view like those of the Moon: In the phases of Venus the planet presents a small "full" image when it is on the opposite side of the Sun. It shows a larger "quarter phase" when it is at its maximum elongations from the Sun. Venus is at its brightest in the night sky and presents a much larger "thin crescent" in telescopic views as it comes around to the near side between the Earth and the Sun. Venus is at its largest and presents its "new passes" when it is between the Earth and the Sun. Since it has an atmosphere it can be seen in a telescope by the halo of light refracted around the planet.
Venus was known in the Hindu Jyotisha since early times as the planet Shukra.
In the West, before the advent of the telescope, Venus was known as a 'wandering star'.
Several cultures historically held its appearances as a morning and evening star to be those of two separate bodies. Pythagoras is usually credited with recognizing in the sixth century BC that the morning and evening stars were a single body, though he thought that Venus orbited the Earth. When Galileo first observed the planet in the early 17th century, he found that it showed phases like the Moon's, varying from crescent to gibbous to full and vice versa. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the solar system was concentric and centered on the Earth.
There is a spacecraft operating in orbit around the planet now. Venus Express probe was designed and built by the European Space Agency. Launched on November 9, 2005 by a Russian Soyuz-Fregat rocket it successfully assumed a polar orbit around Venus on April 11, 2006. The probe is undertaking a detailed study of the Venusian atmosphere and clouds, and is also mapping the planet's plasma environment and surface characteristics, particularly temperatures.
As one of the brightest objects in the sky, Venus has been known since prehistoric times and as such has gained an entrenched position in human culture. It is described in Babylonian cuneiform texts such as the Venus tablet of Ammisaduqa, which relates observations that possibly date from 1600 BC. The Babylonians named the planet Ishtar , the personification of womanhood, and goddess of love.
The Ancient Egyptians believed Venus to be two separate bodies and knew the morning star as Tioumoutiri and the evening star as Ouaiti.
Likewise believing Venus to be two bodies, the Ancient Greeks called the morning star, Phosphoro, the "Bringer of Light". The evening star they called Hesperos, the "star of the evening", but by Hellenistic times, they realized the two were the same planet. Hesperos would be translated into Latin as Vesper and Phosphoros as Lucifer ("Light Bearer"), a poetic term later used to refer to the fallen angel cast out of heaven.
The Romans would later name the planet in honor of their goddess of love, Venus, whereas the Greeks used the name of her Greek counterpart, Aphrodite (Phoenician Astarte).
Venus was important to the Maya civilization, who developed a religious calendar based in part upon its motions, and held the motions of Venus to determine the propitious time for events such as war. They named it Noh Ek', the Great Star, and Xux Ek', the Wasp Star. The Maya were aware of the planet's synodic period, and could compute it to within a hundredth part of a day.
The Maasai people named the planet Kileken, and have an oral tradition about it called The Orphan Boy.
Venus is important in many Australian aboriginal cultures, such as that of the Yolngu people in Northern Australia. The Yolngu gather after sunset to await the rising of Venus, which they call Barnumbirr. As she approaches, in the early hours before dawn, she draws behind her a rope of light attached to the Earth, and along this rope, with the aid of a richly decorated "Morning Star Pole", the people are able to communicate with their dead loved ones, showing that they still love and remember them. Barnumbirr is also an important creator-spirit in the Dreaming, and "sang" much of the country into life.

December 30th 2008

The prominent constellations of winter are now rising in the early part of the evening, Taurus, the Bull, illustrated above, being among the first.
Taurus is marked by a V-shaped pattern of stars that outlines the bull's face. Bright red Aldebaran, the "eye" of the bull, stands at one point of the V. This pattern is part of a cluster of stars called the Hyades — the second-closest star cluster to Earth. It consists of several hundred stars that lie about 130 light-years away.
Aldebaran outshines all the other stars that outline the bull's face. But Aldebaran isn't a member of the Hyades cluster — it just lies in the same direction. It's about 70 light-years away, half as far as the stars of the Hyades. Aldebaran is a red-giant — an old, bloated star that's used up most of its nuclear fuel. It's much larger and much brighter than our own middle-aged Sun.
On the other side of the sky, Venus is shining bright in the west.