Looking Up: Twinkling stars and a comet

Peter Becker

Most of us can recite at least part of the old nursery rhyme “Twinkle, twinkle little star,” whether we like to admit it or not.

Stars that seem to quickly twinkle as we gaze upon them are only a sign that the blanket of air over our heads is a bit turbulent tonight. Passengers on that airliner going overhead know the feeling; from the ground we just enjoy the view and if we like, sing the rhyme.

The scientific word for star twinkling is scintillation. Turbulent nights are not ideal for high magnification at the telescope. Fine detail on planets, the moon or in separating stars that are close together, becomes lost in space. Stars appear to us effectively as points of light; their light is bent, or refracted this way and that as it passes through changing density of air. You will notice twinkling more with bright stars close to the horizon, where there is a thicker layer of atmosphere between you and the star.

Planets in our solar system, however, are noticeably larger in a telescope, being much more than a point of light. Scattered, random points across the disc of the planet are twinkling in turbulent air, blurring the image. To the unaided eyes, however, the many tiny points cancel each other out and the planet generally appears not to twinkle. That’s one way we can tell just by looking up which of those bright stars is really a planet.

Comet Hartley

This week, a fairly dim comet may be seen with binoculars passing the easily-found W-shaped constellation Cassiopeia. Comet Hartley-2 is gradually brightening, but it is only about magnitude +7, well within binocular range but just under the usual limit for unaided eyes.

The comet appears as a small, dim patch and may be tricky to find since the light is spread out and not a concentrated point.

A detailed star map may be found at under “This Week’s Sky at a Glance.” Cassiopeia stands high up in the northeast in early October evenings. The comet is situated immediately right of the “W” shape of five bright stars. On Friday, Oct. 7, see the comet pass very close to the marvelous “Double Cluster,” a pair of star clusters visible to unaided eyes as a fuzzy patch just below Cassiopeia. The view in a binoculars should be impressive.

The comet is expected to reach a height of +5th magnitude over the next three weeks, barely visible to unaided eyes from a clear, rural location.

New moon is on Oct. 7.

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Keep looking up!