Observing overhead this October


Observing overhead this October

Sat, 07 Oct 2017

By Dr. Timothy Bratton

Professor Emeritus of History and Amateur Astronomer

At 7:08 a.m. CDST on Oct. 5, Venus and Mars will be less than 0.25 degree apart, their closest encounter since November 1995.

The only bright planets in the night sky this month are Jupiter and Saturn, but both are sinking toward the sun and will disappear soon. But Venus, Mars and the moon will encounter each other in the predawn sky, so look for them if you arise early enough. A major meteor shower occurs at the end of October’s third week.

Thursday, Oct. 5: When civil twilight begins at 7:08 a.m. CDST, dazzling white Venus (magnitude –3.9) will be 15.93 degrees above the E-ESE skyline, 11 seconds of arc in apparent diameter, 91.6 percent illuminated, 140,748,385 miles from the Earth, and 66,779,450 miles from the sun.  Use binoculars, field glasses or a telescope to spot orange-red Mars (mag. 1.8) just 0.23 degree to its lower right. This is the closest conjunction between the two planets since November 1995! The “Red Planet” will be then be just 3.7 arc-seconds across, 98.6 percent lighted, 154,873,670 miles from the sun, and 235,736,485 miles from Earth. Both planets are emerging from behind the sun, so they are both nearly “full” while appearing smaller than usual because of their great distances from our world. Venus outshines Mars by 189.73 times.

Friday, Oct. 6: Full moon occurs today (Friday, Oct. 5) at 1:41 p.m., when it will be 4 degrees beneath the northern horizon, 31.6 arc-minutes in apparent span and 232,043 miles distant. This is also the “Harvest Moon,” the full moon closest to the autumn equinox, which fell on Sept. 22 this year. Because the ecliptic (the path of the Earth around the sun) runs almost parallel to the horizon in the autumn, the moon rises only about 30 minutes later each evening instead of the usual 50 minutes. The “Harvest Moon” looks especially big (an optical trick called “the moon illusion”) and yellow (as it is dimmed by atmospheric refraction) since it is so low on the skyline. Farmers used the light from the “Harvest Moon” to gain a few more hours for gathering in their crops. This full moon is also nicknamed the “Fruit Moon.”

Monday, Oct. 9: The waning gibbous moon will reach perigee, its closest distance to the Earth this month, at 12:53 a.m., when it will be 227,942 miles away, 35.87 degrees above the ESE horizon, 84.44 percent lit and 32.57 arc-minutes across.

Thursday, Oct. 12: Last quarter moon takes place at 7:26 a.m., when Luna will be 62 1/3 degrees over the southern horizon, 32.2 arc-minutes in diameter and 230,615 miles distant.

Sunday, Oct. 15: At 6 a.m. the waning crescent moon will be 27.25 degrees above the E-ESE skyline, 31.5 arc-minutes in apparent span, 19.63 percent illumined and 235,560 miles away.  The bright blue-white star just 0.55 degree above it is Regulus (Alpha Leonis, mag. 1.35), the most noticeable sun in the constellation of the Lion. Regulus also marks the bottom of the “Sickle” or reversed “Question Mark” asterism within Leo.

Tuesday, Oct. 17: At 6 a.m. the slender crescent moon will have risen only 5 1/3 degrees above Jamestown’s eastern horizon. Luna will be then 31 arc-minutes across, 5.88 percent sunlit and 239,518 miles away.  Red-orange Mars (mag. 1.8) will lie then 1.45 degrees to the moon’s upper right; use optical aid to detect it in the predawn haze. At this time the “Red Planet” will be 3.8 arc-seconds in span, 98.03 percent illumined, 154,827,190 miles from the sun and 230,809,270 miles from Earth.

Thursday, Oct. 19: New moon occurs at 2:13 p.m., when it will be 38.1 degrees above the S-SSW skyline, 30.37 arc-minutes across and 244,501 miles distant. As it will be passing then 3.82 degrees north of the sun, no solar eclipse can take place this month.

Saturday, Oct. 21: The Orionid meteor shower should attain its climax this morning, although some stragglers from this stream might be visible until Nov. 7. These objects are debris from Halley’s Comet, which replenished the swarm last when it passed this way in 1985-86. Since Halley’s Comet has been observed since at least 239 B.C., it has had plenty of time to distribute debris in its wake, so that virtually every part of its orbital path is scattered with rubble. This has made the Orionids among the most reliable of all meteor showers. Because they approach the Earth almost head on, these rapidly moving objects enter the atmosphere at an average speed of 41 miles per second. Usually about 10 to 20 meteors per hour are visible if moonlight does not interfere; since the young crescent moon sets not long after the sun on Friday evening, observing conditions should be nearly ideal this year if the weather cooperates. Perhaps one of every two Orionids leaves a smoky trail behind that persists for 1 to 2 seconds; you may glimpse such a train twisting in the winds of Earth’s upper atmosphere (use binoculars to see this more clearly). Some of these meteors are colored or explode into fireballs. By 4 a.m. the Orionid radiants will have risen approximately 50.5 degrees above the SE horizon. At this hour “shooting stars” will appear to come from two or more separate foci in the upraised club of Orion, about 10.5 degrees to the upper left of the bright red star Betelgeuse (Alpha Orionis, magnitude 0.5).

Tuesday, Oct. 24: The moon reaches apogee, its farthest distance from the Earth this month, at 9:26 p.m. At that moment Luna will be 251,751 miles away, just 4.13 degrees above the SW-WSW skyline, 29.5 arc-minutes in apparent span and 24.57 percent sunlit.

Friday, Oct. 27: First quarter moon transpires at 5:22 p.m., when it will be 18.75 degrees above the SE-SSE skyline, 29.85 arc-minutes across, and 248,726 miles from our planet.

Tuesday, Oct. 31 (Halloween): The waxing gibbous moon will be shining tonight, which will provide extra light for trick-or-treating. Halloween is the “cross-quarter” day halfway between the first days of autumn and winter. With sunlight obviously dying, this particular cross-quarter day would have been the one most closely associated with the powers of darkness.