Fall Planet Guide: Saturn is Best Bet

Have you ever heard of the word "combust?"  As defined in my American Heritage Dictionary of the English Language, it means, "Not visible because of proximity to the Sun."

And as it turns out, during the coming days and weeks, four out of the five bright naked-eye planets will indeed become combust!  This is a rather unusual situation, since there are usually at least two bright planets in view on any given night.  But by the last week of October, only one out of the five bright planets will be visible. 

Already, Venus and Mars are out of view, hidden by the brilliant solar glare.  Jupiter and Mercury, currently in the evening sky, will gradually slide into invisibility during late October.  Only Saturn will be readily visible all through the month of October in a dark sky . . . although you'll have to get up during the wee hours of the morning to get a good look at it.

Here is a summary of what you can see (or not see) regarding the five brightest planets during these next several weeks:

MERCURY-begins October shining at magnitude -0.1-equal in brightness to the similarly hued star Arcturus in Bootes.  Yet, it will probably require optical aid as it sinks toward the horizon after sunset. 

Look for it very low in the west-southwest about 30 minutes after sunset during the first half of October.  You'll need a ton of luck, to actually see it, however, since this tiny, rocky world is having its worst apparition of 2006 for viewers located near and along latitude 40 degrees north.  Although it attains a greatest elongation of 25-degrees on Oct. 17, its southerly location in Libra lays it low for us northerners.  Jupiter will be passing well above and to Mercury's right in the evenings that immediately follow. 

But by the final week of October, you can probably forget about Mercury; by then it will be so low and will have become so faint that very few viewers north of the equator will be able to see it. 

In stark contrast, for those living south of the equator, Mercury will put on a great evening show; appearing to stand almost straight up above the setting Sun in mid-October and remaining above the western horizon until after evening twilight has ended.

On Nov. 8-9, Mercury will arrive at inferior conjunction, crossing directly between the Earth and the Sun and resulting in a rare "transit" across the Sun's disk.  For favorably placed viewers in North and South America, the Pacific Ocean, Australia, New Zealand and eastern Asia, Mercury will appear as a tiny black dot crossing the face of the Sun.  It will be this planet's last transit until the year 2016.  More details on how to view this unusual sight will appear on SPACE.com in the coming weeks, so be sure to stay tuned! 

VENUS-is completely out of sight in October; it arrives at superior conjunction behind the Sun on Oct. 27.  Venus will emerge into view as the bright Evening Star come early-to-mid December. 

MARS-like Mercury and Venus is hidden deep in the Sun's glow during October.  It will be in conjunction with the Sun on Oct. 23.  Don't even think about looking for it in the morning sky until December. 

JUPITER-sets about 100 minutes after the Sun at the start of October, but less than 45 minutes by month's end.  This big world is unfortunately, too low at dusk for good telescopic observations.

My guess is that Oct. 24 will be-realistically-the last evening that most people will be able to see it.  Binoculars will help a lot.  If somehow you do locate Jupiter, then also try for the exceedingly thin crescent Moon about 10-degrees to the left of Jupiter albeit somewhat lower, and much-dimmer Mercury, about 4-degrees below and to Jupiter's left.  Keep in mind that your clinched fist, held at arm's length is roughly equivalent to 10-degrees; so the Moon and Jupiter will be separated by approximately "one-fist."

Binoculars will also show Jupiter pulling well away from the star Zubenelgenubi early in the month.

SATURN-is the only bright planet that is well placed for viewing in October.  At midmonth it rises soon after 2 a.m. local daylight time and is well up in the eastern sky at dawn. 

Saturn creeps 2.5-degrees eastward in Leo, and will close the month 6-degrees west of the bluish first-magnitude star, Regulus.  Of magnitude +0.5, Saturn is outshone only by similarly hued Procyon among all those stars located within 30-degrees of the planet.  The rings continue to slowly close to our line of sight; telescopic observers will see the rings south side tilted 13.7-degrees toward us at the start of the month, but 12.7-degrees by month's end.  

On the morning of the 16th, Saturn will sit below and slightly to the right of a fat crescent Moon.

Basic Sky Guides

  • Full Moon Fever
  • Astrophotography 101
  • Sky Calendar & Moon Phases
  • 10 Steps to Rewarding Stargazing
  • Understanding the Ecliptic and the Zodiac
  • False Dawn: All about the Zodiacal Light
  • Reading Weather in the Sun, Moon and Stars
  • How and Why the Night Sky Changes with the Seasons
  • Night Sky Main Page: More Skywatching News & Features

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

DEFINITIONS

1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).