Comet ISON brightens before dawn

Mars points the way to this celestial visitor during October’s first half.
By | Published: October 1, 2013 | Last updated on May 18, 2023
Comet ISON on September 24
Astroimager Damian Peach from Hampshire, England, captured Comet ISON (C/2012 S1) on September 24, 2013, with a 17-inch corrected Dall-Kirkham reflector and an FLI PL-6303e CCD camera. He combined five 3-minute luminance images with 2-minute images through red, green, and blue filters.
Damian Peach
October should prove to be a harbinger of things to come. Will Comet ISON (C/2012 S1) shine brightly enough to see with naked eyes when it passes just 1° from the Sun November 28? Will this celestial visitor produce a gleaming head and a long flowing tail during November’s latter half and December’s first two weeks? If ISON lives up to expectations during October, astronomers predict that it will deliver on its promise of becoming a Great Comet later this year.

Many observers will get their first looks at this comet through telescopes during the first half of October. It shines around 11th magnitude as the month begins and should brighten by another magnitude (a factor of 2.5), at least, by mid-October. This puts it within the range of a 6-inch telescope under a dark sky. Fortunately, the Moon cooperates by staying out of the morning sky throughout the month’s first two weeks.

Your best guide to finding the comet during this period is Mars. The Red Planet begins the month just 2° (four times the Full Moon’s diameter) south of ISON, a gap that drops to 1° by the 15th (use the finder chart below as a guide). Not only is Mars bright, shining at magnitude 1.6, but it also has a distinctive orange-red color that makes it easy to find on any clear morning. Once you center the planet in a low-power field of view, gently nudge your scope northward to locate ISON.

The appearance of ISON near Mars is not simply a line-of-sight coincidence — the two bodies actually reside near each other in space. They make their closest approach October 1, when the comet whizzes 6.7 million miles (10.8 million kilometers) from the planet.

Keep a watch on the morning sky during October’s first two weeks, and you can’t help but notice that Mars and its cometary companion are approaching a conspicuous star. This is magnitude 1.4 Regulus, Leo’s brightest sun and a near match to Mars in terms of brightness. The star’s subtle bluish hue, however, contrasts nicely with the ruddy planet.

The seemingly lockstep motions of Mars and ISON carry them to within a stone’s throw of Regulus on October 15. The three objects form a spectacular straight line that morning, with Mars 1° north of Regulus and ISON 1° north of the planet. The trio rises shortly before 3 a.m. local daylight time and climbs one-third of the way to the zenith by the time morning twilight starts around 5:30 a.m. ISON picks up speed later in the month and will leave the star and planet behind as it moves closer to the Sun during the next several weeks.

Follow Comet ISON’s travels at, and pick up the October issue of Astronomy and the special issue The Great Comet of 2013 for even more coverage of ISON’s voyage to the inner solar system.

Comet ISON October finder chart
Comet ISON crosses Leo in October, following a similar track to Mars but moving slightly faster. The map shows stars as faint as magnitude 8.7.
Astronomy: Roen Kelly