SKY HAPPENINGS

The Sky for October 2013
By Faylene Roth

Find the loneliest star in our night sky by locating the NW corner of the Great Square of Pegasus. Draw a line to the SW corner of the Square and continue south—through faint Aquarius—to lone 1st magnitude star Fomalhaut (30° S of the celestial equator) in the constellation Piscis Austrinus. At midnight face north and find Ursa Major sitting upright low on the horizon.

DAYLENGTH
Without missing a beat, the sun rises one minute later each morning during the month of October. Days shorten more quickly at sunset, which arrives 41 minutes earlier by the end of the month. The lingering twilight effect of summer nights also ceases—delivering a 10 hour 35 minute day by month’s end. Twilight fades quickly now. Thirty minutes of sufficient light during civil twilight allows time to complete outdoor chores. Then nautical twilight transitions quickly to a dark landscape in the next 30 minutes. Overhead skies, too, darken rapidly as astronomical twilight erases ambient light from the horizon. The skies are dark one and one-half hours after sunset. Stargazing is at its best. The reverse progression applies to dawn.

AUTUMN EQUINOX
An imaginary line called the ecliptic traces the path of the sun across the sky relative to the background stars. Earth’s orbit around the sun creates the seasons because the equator is tilted from the plane of the ecliptic at an angle of 23.5 degrees. The angle and direction of the tilt does not change as the earth travels around the sun, but it does present a different face of the earth towards the sun. In winter the northern hemisphere tilts away from the sun; in summer it tilts towards the sun. In spring and fall—at the equinox—both hemispheres receive the same amount of light because the sun shines perpendicular to the equator. An extension of the earth’s equator into the celestial sphere creates the celestial equator. The equinox occurs at the point in the earth’s orbit where the plane of the ecliptic intersects the celestial equator. This year the intersection occurs on September 22 at 2:44pm MDT. Sunrise will be due east and sunset will be due west. The length of day and night would be equal if it were not for the effect of refraction which means that light bends around the curvature of the earth, making the sun visible before it crests the horizon.

MOON HAPPENINGS
October 4 – New Moon occurs at 6:34pm.
October 11 – First Quarter Moon sets soon after midnight.
October 18 – Full Moon occurs 5:38pm and rises at 6:28pm.
October 26 – Last Quarter Moon rises soon after midnight.
(The time of moonrise and moonset assumes a flat horizon. Actual time may vary.)

ZODIACAL LIGHT
The zodiacal light continues to be visible in October. It is best seen in the morning sky before astronomical twilight brightens the eastern horizon (1-1/2 hours before sunrise). The zodiacal light is a cone of white light emanating from the sun before it crests the horizon. It extends almost 60° into the sky on a line almost perpendicular to the horizon. The light illuminates dust particles in space between earth and sun.

METEOR EVENTS
October offers numerous meteor showers, although none of them are major meteor producers. The Draconids are active October 6-10. October 7-8 provides the best opportunity to view up to 10 meteors per hour. Occasionally, this shower proves to be a bonanza that produces hundreds of meteors, so it is worth watching. A circumpolar constellation, Draco is high in the sky early in the evening; however, viewing still improves after midnight. The Piscid Meteor Shower produces up to 15 meteors per hour throughout the month radiating from the Pisces-Aries border beneath Pegasus. The Geminids and the Orionids are most active October 19-22. Meteors will be seen in all regions of the sky for all these meteor showers. Their sources can be determined by tracing their path backwards to the originating region of the sky, hence their names.

Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5 ̊. Middle, ring, and little finger touching represent about 5 ̊. The width of a fist is about 10 ̊. The fist with the thumb extended at a right angle equals 15 ̊. The hand stretched from thumb to little finger approximates 20 ̊-25 ̊. The diameter of both the full moon and the sun spans only 0.5 ̊. Adjust for the size of your hand.

Primary Sources: USGS; U.S. Naval Observatory; Your Sky at http://www.fourmilab.ch/yoursky/
To find out when the space shuttle and International Space Station are visible from your location, go to: http://spaceflight.nasa.gov/realdata/sightings/index.html and click on Sighting Opportunities.

VISIBLE PLANETS

Jupiter rises around midnight and appears high in the overhead sky in morning twilight. Look for it about 7° SW of the twin stars (Pollux and Castor) in Gemini. On the night of October 24/25 a waning gibbous moon rises about one-half hour (seven degrees) ahead of Jupiter. The following night Jupiter rises before the moon. (Magnitude -2.1)

Mars begins October below the lion’s head in Leo. On October 15 it rises within 1° of 1st magnitude star Regulus, the heart of the lion. Mars rises (on the horizon) around 3:00am. Look for its small red-orange disk high in the eastern sky in early morning twilight. (Magnitude +1.6)

Saturn sets in the evening twilight with Libra. By month’s end it disappears into the sun’s glare as it approaches conjunction with the sun (its pass around the far side of the sun). (Magnitude +1.2)

Venus rises earlier each day (before noon) which positions it a little higher in the western sky each evening. On October 8 Venus, Antares, and a waxing crescent moon form a triangle above the western horizon. Midmonth Venus sweeps quickly through Scorpius as it passes from Libra to Ophiucus. On October 16 Venus appears within 1.5° of the red-giant Antares (Scorpius). (Magnitude -3.9)

Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.

Hold the star chart high above your head and match the compass directions to the direction you are facing. Adjust the star chart by orienting Ursa Major (Big Dipper) to match its position in the sky.

The star chart approximates the sky from astronomical twilight to midnight. As the night and month progresses, the constellations shift toward the northwest. The celestial equator is measured in hours (h). The ecliptic is measured in degrees.