Moon, Saturn & Star Form Last Evening Sky Triangle for Now

The ringed planet Saturn will soon say goodbye to the evening sky for awhile, but only after taking one final bow this week with two companions: the moon and the bright star Spica.

Saturn and the moon have formed triangles in the night sky with a star several times this year, but this week the planet forming up for the last time before it disappears in the twilight sky for a bit. The view, weather permitting, could be striking.

Take this past weekend, for example, when I was in Roxbury, N.Y. attending Starfest, an annual affair held by the Amateur Observers' Society of New York in the Catskill Mountains. Scattered showers and thunderstorms during Friday cleared away by late at night to allow observers to view some of the delights of the summer sky. 

Saturday was bright and sunny and after sundown as the sky darkened, many of us turned our attention to the west where a large isosceles triangle of three bright stars appeared to sit protrude up from the horizon.

At the top or vertex of the triangle gleamed the orange-yellow star, Arcturus, next to Sirius, the brightest star visible from mid-northern latitudes. Well below Arcturus were two other stars hovering low above the west-southwest horizon. 

On the left was the bluish Spica, brightest star of the constellation Virgo. The other star however, was not a star at all, but a planet: the ringed wonder of our solar system, Saturn. It was about this same time, that many of the various instruments that had been brought by Starfest participants were being trained on Saturn to get a view of it before it slipped too low into the horizon haze.  

This sky map of Saturn, the moon and Spica shows how they will appear this week.

Saturn's stunning summer

Of course, because of its beautiful rings, Saturn is always a prime target for telescopes. As little as a magnification of 30-power will allow you to get a glimpse of the rings in a small telescope. Two years ago, however, this wasn’t possible because the rings were turned edge-on toward us; seeing the rings even in a large telescope was a difficult-to-nearly impossible task. 

Now, however, the rings are now gradually opening up to our earthly perspective, their north face is currently tilted about 8 1/2 degrees in our direction and by year's end that angle will have increased to nearly 15 degrees. So the views of Saturn will only be getting better as time wears on.

Saturn has been a convenient evening object since early spring, but its days as an evening object are drawing to a close. Indeed, in a few more weeks, Saturn will be much lower and more deeply immersed in the western twilight sky and will be more difficult to see. 

And by the beginning of September it will be all but gone from view, not reappearing again until early fall in the morning sky before sunrise.  So you should try to catch it now before it leaves the evening sky.

How to see Saturn this week

You'll have a good landmark in the sky to help identify Saturn on both Wednesday (Aug. 3) and Thursday evenings: the moon. On Wednesday evening, look toward the west-southwest sky at dusk to find the waxing crescent moon; about 10 degrees above and slightly to the left of the moon you'll see a bright star shining sedately with a yellow-white luster. Remember, your clenched fist held at arm’s length measures roughly 10 degrees.

That "star" is Saturn. And about a dozen degrees to the left of Saturn will be the bluish star Spica. This triangular array of the moon, a star and a planet will be visible until about 11 p.m. local daylight time, when the moon drops below the horizon.

Then on Thursday evening (Aug. 4), the arrangement will have changed. 

Since the moon is so much closer to us compared to Saturn and Spica, it will have moved to a position about 4 degrees directly below Spica. Now we have a different triangle — a right triangle — with the right angle at Spica and Saturn appearing well off to the west (right) of the moon and star.

This new configuration should remain in view until about 11:30 p.m. local time when the trio drop low into the horizon haze and ultimately disappear beyond the horizon.

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.