The end of the Southwest monsoon season signals the return of cool, clear autumn skies. The bright summer constellations are setting as darkness falls and the winter constellations begin to rise after dark, Taurus leading the way.
The Pleiades in Taurus is visible between tree branches. This star cluster rises about 9pm MDT on October 1st.
Between the bright constellations of summer and winter lie faint autumn constellations, like Aquarius, Pisces, and Aries. But, while autumn stars may be fewer and fainter, there are still wonderful sights to see in October, with the naked eye, binoculars, and telescopes.
The Milky Way itself stretches from horizon to horizon, passing directly overhead. Venus is brilliant in the WSW after sunset, while Mercury climbs the dawn sky in the east. Jupiter and Saturn grace October skies, just east of south. Uranus is barely visible to the unaided eye, rising shortly after sunset.
Mercury becomes more easily visible as it rises in the east during dawn. It reaches greatest western elongation from the Sun on October 26th.
At magnitude -4.4, Venus continues to dominate the western skies during twilight and just after dark. It reaches greatest eastern elongation from the Sun on the 29th, appearing like a tiny first quarter moon in a small telescope.
Jupiter is the brightest object in the southern skies during twilight and after dark, still brilliant as it recedes from Earth after its opposition in August. A small telescope can reveal Jupiter’s four Galilean moon, endlessly orbiting the giant planet.
Like Jupiter, Saturn is still an easy object in the south after sunset, also receding from Earth after its closest approach in August.
As Uranus approaches opposition (November 4th), it’s visible for most of the night, rising just after dark in early October. At magnitude 5.7, this large outer planet is barely visible to the unaided eye on clear, dark nights. A star map is needed to identify it in the constellation Aries. Uranus will appear as a small, featureless, blue disk in small to medium telescopes.
October provides an opportunity to familiarize yourself with some of the lesser-known constellations visible in our skies. One of these is Cetus (pronounced, SEET-us), the fourth largest constellation in the entire sky, rising shortly after dark on the first of the month and during twilight at month’s end.
In Greek mythology, Cetus was a sea monster. Today, it’s mostly known as a whale. Visually, the constellation appears to have a diamond-shaped circle of stars, located beneath neighboring Aries, and a large ‘body’ below and to the west, connected by a long ‘tail.’ Depictions of the whale usually show the diamond-shaped circle of stars to be tail flukes, with the length of the tail connecting to the whale’s body and head.
The constellation Cetus is often known as ‘the whale,’ with tail flukes on the left and the main portion of the tail in the center, connecting to the whale’s body and head on the right. The outline of the constellation is mostly comprised of third and fourth magnitude stars, making it a challenging constellation to identify.
Cetus is one of the ‘water constellations’ in this part of the sky. The fall water constellations include Delphinus (the dolphin), Capricornus (the sea goat), Aquarius (the water carrier), Pisces (a pair of fish), Piscis Austrinus (the southern fish), and Eridanus (the river), in addition to Cetus (the whale).
Exactly what the water constellations signify is unclear. Some say the term ‘water constellations’ refers to the section of the sky that’s vague and dim, like a pool of water. Some say they signify rainy seasons in certain parts of the world. Others suggest that the water constellations have some kind of connection to an ancient flood in the Tigris-Euphrates basin in western Asia.
Suffice it to say, the water constellations represent an area of the sky in which we look away from our galaxy, outward, into the universe at large.
Cetus is not one of the thirteen constellations of the zodiac (the constellations that the Sun passes through as the Earth orbits it – the Sun’s path amongst the stars is known as the ‘ecliptic’). However, the ecliptic almost clips a corner of the constellation. The Moon and the planets loosely follow the ecliptic, but often run slightly above or below it. As a result, the Moon and planets sometimes enter Cetus for brief periods of time.
At magnitude 2.0, Beta Ceti is brighter than Alpha Ceti (the brightest star in a constellation is usually designated ‘alpha’). Known as Diphda and, sometimes, Deneb Kaitos, Beta Ceti is the brightest star in Cetus. Marking the whale’s mouth, Diphda is a red giant star that has exhausted its hydrogen and is now burning helium. It’s also a variable star, meaning that its brightness varies over a period of time.
Alpha Ceti, magnitude 2.5, is the second brightest star in Cetus and is also a variable, red giant. Known as Menkar, this star marks one of the whale’s tail flukes.
The most interesting star in Cetus is Omicron Ceti or Mira (MEE-ra), yet another variable, red giant star. However, Mira, Latin for ‘astonishing,’ is no ordinary star, as its name implies.
To the naked eye, the star actually appears, disappears, and reappears over a period of 11 months. At its brightest, Mira’s magnitude can vary between 2.0 and 4.6. At its dimmest, Mira’s magnitude ranges between 8.6 and 10.1, well beyond naked eye visibility, even under the best of conditions.
As Mira varies in brightness, it expands and contracts and changes temperature. Near periods of maximum brightness, Mira is compact and hot. As the star pulses, it sheds gas and dust. Since, like all stars, Mira moves through space, the material shed by the star trails out behind it. Mira’s very faint tail is more than 76 trillion miles long. This is truly an astonishing star!
An ultraviolet image of Mira’s bow shock (right) and tail, taken by NASA’s Galaxy Evolution Explorer space telescope. Courtesy of the California Institute of Technology.
But, wait, there’s more! Mira also has a companion star. At magnitude 10.3, Mira’s companion is much smaller and dimmer than the primary star. These two stars revolve around a common center of gravity.
Mira is currently visible to the unaided eye. It just passed maxima (maximum brightness) in August. The photo used to create the map of Cetus (taken September 29th) clearly shows Mira.
Since Cetus is in an area of the sky unencumbered by the dust clouds of the Milky Way, numerous, distant galaxies can be seen within its borders. Unfortunately, viewing details of these deep sky objects are beyond the reach of most people’s equipment.
Two objects that are within reach of amateur equipment are the barred spiral galaxy Messier 77, magnitude 9.6, and a planetary nebula known as the Skull Nebula (NGC 246), magnitude 10.9. Long exposure photographs of these objects can bring out details that the unaided eye can’t see.
October Night Sky Events:
October 6: New moon.
October 7-8: The Draconid meteor shower peaks on the night of the 7th and pre-dawn of the 8th. Under optimal conditions, about 10 meteors per hour might be seen during the peak. The waxing crescent moon won’t interfere with viewing. The shower runs from October 6-10.
October 14: After sunset, view the waxing gibbous moon between the planets Jupiter (left) and Saturn.
October 20: The full Hunter’s Moon occurs at 8:57am MDT, near the Pisces-Aries border.
Last year’s Hunter’s Moon, setting behind the Jemez Mountains. The distortion of the Moon was caused by atmospheric turbulence.
October 21-22: The Orionid meteor shower peaks on the night of the 21st and pre-dawn of the 22nd. Under optimal conditions, about 20 meteors per hour can be seen during the peak, but the waning gibbous moon will hamper the viewing of fainter meteors this year.
October 25: Mercury reaches greatest western elongation from the Sun and is visible in the dawn sky in the constellation Virgo.
October 29: Venus reaches greatest eastern elongation from the Sun and is visible near the borders of Scorpius, Ophiuchus, and Sagittarius. The planet sets in dark skies around 8:30pm MDT. Since Venus is at elongation, it appears to be 50% illuminated, shining at magnitude -4.4.