On Oct. 24, an inconspicuous comet visible only through a powerful telescope startled sky watchers by brightening by a factor of more than 1 million times to become easily visible to the unaided eye. Comet Holmes was first spotted in 1892 when it underwent a similar, though much less dramatic, brightening, but it has been steadily decreasing in brightness on each return of its roughly seven-year period until this latest eruption.

No astronomer could have predicted, nor easily explain, how a comet 25,000 times too faint to see with the naked eye managed to increase 1 million times in brightness over less than two days. The best explanation is that a large piece of the nucleus, which is only about 2.2 miles in diameter, broke away and immediately disintegrated into a surrounding coma of dust and gases. The rapid expansion of the coma accounts for the increase in size and brightness.

By mid-November astronomers at the University of Hawaii reported that comet Holmes “continues to expand and is now the largest object in the solar system.” On Nov. 9, comet Holmes’ coma measured 869,900 miles in diameter compared to the sun’s average diameter of 864,900 miles.

In terms of mass, of course, the coma weight is negligible compared to that of the sun. Comet Holmes should be visible for several weeks as a small luminous “puffball” moving slowly through the constellation Perseus (see star chart).

Focus on the planets

Mercury is lost in the glare of the sun during December.

Venus blazes in solitary splendor high in the southeast at dawn where it will remain throughout the month.

Mars is the major attraction among planets in December appearing bigger and brighter than it will for nearly another nine years. Mars is high in the east around 9 p.m. as the month opens but look for it to rise at sunset, and remain up all night, as the year draws to a close.

Jupiter will peek briefly above the southwest horizon at twilight during the first week of December but will soon be lost in the sun’s glow.

Saturn rises in the east among the stars of Leo after midnight to start the month but will be visible by 10 p.m. at month’s end. Viewers hoping to see Saturn’s fabled ring system will be disappointed as they lie as close to edge-on with respect to Earth as they have in a decade. However, Saturn’s major moons will still be on display.

Uranus in Aquarius and Neptune in Capricornus are visible through powerful binoculars with aid of a finder’s chart. Go to the Web site

skyandtelescope.com/uranusneptune for directions on finding these two distant planets.

December events

1 Sunrise, 6:52 a.m.; sunset, 3:56 p.m. Moon in last quarter, 7:44 a.m.

5 A waning crescent moon, Venus, and the bright star Spica form a triangle in the southeast about an hour before sunrise.

6 The moon is at apogee, or farthest distance from Earth, today.

7 The earliest sunsets of the year occur around this date even though the longest night of the year is Dec. 21. The difference is in the changing time of sunrise, which comes latest in early January.

9 New moon, 12:40 p.m.

14 The Geminid meteor shower peaks around this date with very little moon to interfere with the display. Look for the Geminids originating near Castor in the constellation of Gemini.

17 Moon in first quarter, 5:17 a.m.

18 The sun enters Sagittarius on the ecliptic. Mars is closest to Earth for the year at 54.8 million miles away.

22 Winter solstice, 1:10 a.m. This is the point where the sun reaches its most southern point below the celestial equator marking the first day of winter in the Northern Hemisphere. The sun enters the astrological sign of Capricornus at the solstice but astronomically is still in Sagittarius. The moon is at perigee or closest approach to Earth. During the evening hours the moon shines in the east with Capella to its left, Aldebaran to its right and Mars to its lower left.

23 Full moon, 8:16 p.m. The full moon of December is known variously as the Cold Moon, Long Night Moon, and the Moon Before Yule. Note that during the evening the moon passes extremely close to Mars and, in some parts of the world, actually passes in front of, or occludes, the planet.

25 Merry Christmas!