October, the 10th month, is named after ‘octo,’ the Latin word for eight. Remember, the ancient Roman calendar started in March until Pope Gregory XIII reformed the calendar in 1582, establishing January as the first month of the year.

Focus on the planets

Mercury appears low on the eastern horizon about an hour before dawn during the second week of the month. Mercury continues to rise and brighten over the next two weeks, giving northern viewers their best morning sightings of the innermost planet for the year.

Venus is low on the southwestern horizon at dusk and sets about an hour after the sun as October opens. By midmonth, it will be setting with the sun and be lost from view for several weeks.

Mars is fairly well up on the eastern horizon at dawn but will appear dim and hard to find as the sky brightens.

Jupiter rises after midnight and stands well up on the eastern horizon in the hours before dawn. Jupiter’s apparent size grows during October, making this a good month to examine the gas giant’s surface markings and check out its four major moons with any reasonably powerful telescope.

Saturn rises just before midnight and spends the month in the northern portions of the easily identified winter constellation of Orion. Saturn’s open-ring system gives a spectacular view of its most prominent feature.

Uranus and Neptune are located on the southwestern horizon in the constellation of Capricornus. Look for the greenish and bluish disks of Uranus and Neptune respectively during the hours before midnight.

Pluto is lost to view during October.

Our celestial neighborhood

When the Voyager missions visited Jupiter in 1979, they encountered an object that seemed to come right out of a science fiction novel. Io, the fifth of Jupiter’s many moons, looked, as one NASA scientist put it, “like a giant pizza pie.” Giant plumes of sulfur gas released from volcanoes spewing molten sulfur hung above Io’s surface. Io’s dark orange coloration, its surface pocked with volcanoes, and the total absence of impact craters made it unlike any other body ever encountered in the solar system.

Now, 23 years later, NASA’s Galileo mission is taking leave of Io’s neighborhood after having added to its mysteries. Scientists have been able to count more than 120 active volcanoes, making Io the most volcanically active body in the solar system. Often these take the form of intensely hot “fire fountains” that spew sulfurous lava more than a kilometer above the surface. All of this from a body having roughly one-half the diameter of the moon.

But what has planetary geologists puzzled is the fact that Io has several nonvolcanic mountains even though there is no sign of plate tectonic activity. This is a puzzle that will have to be solved by some future mission to Jupiter.

October events

1 Sunrise, 6:33 a.m.; sunset, 6:17 p.m.

2 Jupiter shines to the right of the crescent moon shortly before dawn.

5 The thin crescent moon lies slightly to the upper left of Mercury before dawn. Dim Mars is directly above Mercury.

6 New moon, 7:17 a.m. The moon is at perigee, or closest approach to the earth and the combination of these two events could lead to abnormally high and low tides.

8 The waxing crescent moon is directly above Venus shortly after sunset. Orange-colored Antares is to the moon’s left.

13 Moon in first quarter, 1:33 a.m.

16 The lone star well below the moon tonight is the “autumn star” Fomalhaut.

20 The moon is at apogee, or maximum distance from the Earth, today.

21 Full moon, 3:21 a.m. As the full moon after the harvest moon, it is called the Hunter’s Moon. This should be the peak evening for the Orionid meteor shower; however, the full moon drowns out any hope of seeing the display.

23 The Pleiades star cluster is above, and a little to the left of the moon about 10 p.m. while orange Aldebaran is situated to the moon’s lower left. The sun enters the astrological sign of Scorpio but, astronomically is it still in Virgo.

25 Look for the moon close to Saturn tonight.

27 Daylight-saving time ends at 2 a.m. tonight. Be sure to set your clocks back an hour.

29 The waning quarter-moon shines near Jupiter during the pre-dawn hours.

31 Sunrise, 6:12 a.m.; sunset, 4:26 p.m.

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.