Mercury lingers in the twilight

Mercury makes a brief appearance, as Mars covorts with Saturn. Also, the first day of summer approaches.

Published: June 14, 2006

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June 14, 2006

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WAUKESHA, WI - Night-sky treats abound this week as we near summer's official start. Three planets, each visible to the unaided eye, now grace evening twilight — Mars, Saturn, and elusive Mercury. Mars and Saturn buzz the famous Beehive star cluster in Cancer, while Mercury makes its evening swan song for 2006. Also, June 21 is the summer solstice — the first day of summer and the longest day of the year in the Northern Hemisphere.

Mercury makes a brief appearance in the evening sky. Begin looking about 20 minutes after sunset for a starlike point hovering more than 10° (the span of a fist held at arm's length) above the west-northwest horizon.

Photo by Astronomy: Roen Kelly

Mercury's cameo
Mercury is the planet with the smallest orbit around the Sun, so it always appears in twilight. We view Mercury's orbit almost edge-on from Earth, so, from our perspective, Mercury seems to oscillate from one side of the Sun to the other as the planet orbits. Each time Mercury's travels pull it away from the Sun's glare, skygazers on Earth have a fleeting chance to view Mercury shortly after sunset or before sunrise.

Astronomers refer to Mercury's greatest angle east (visible in the evening) or west (morning) of the Sun as an "elongation." This week and next, Mercury brightens and stands higher in the western sky as it approaches its greatest eastern elongation June 20, when the planet stands 25° from the Sun. This is the best opportunity to see Mercury in the evening sky for the remainder of 2006.

Begin looking for a bright starlike point in the western twilight about 20 minutes after sundown. Mercury will be about 12° above the horizon, or slightly higher than the width of a fist held at arm's length, and glowing at about magnitude 0.2. After June 20, the planet fades quickly, becoming harder to see in the twilight, and begins a fast slide back toward the Sun.

Mercury's orbit
Mercury orbits the Sun at an average distance of only 36 million miles (58 million kilometers). Earth is nearly 3 times as far, so, from our perspective, Mercury always stays near the Sun.

When Mercury is west of the Sun, we view the planet as a "morning star" in the east before sunrise. Some elongations are better for viewing than others because of Earth's tilt and Mercury's out-of-round orbit.

Scope view
Through a telescope, observers will watch Mercury go through phases similar to the Moon's. On June 14, the planet's disk is more than 50-percent illuminated. At greatest elongation, Mercury's disk will appear 38-percent lit. After this, it shrinks to a crescent and fades rapidly.

Most observers detect no surface markings on Mercury. It takes a seasoned observer and excellent atmospheric conditions to see anything at all on the planet, even through the largest amateur telescopes. Experienced amateurs, however, have recorded dusky markings and occasional bright areas on the planet.

Click here for more information on Mercury.

Mars glides through the Beehive cluster (M44) in Cancer June 15. Saturn lies only 1.2° away - a nice scene in binoculars or a rich-field telescope. The two planets are closest June 17.

Photo by Astronomy: Roen Kelly

Mars and Saturn tango
As darkness falls and Mercury slips too low to see easily, look for a pair of bright stars above the planet. These are Pollux (left, magnitude 1.1) and Castor (right, magnitude 1.6), the brightest stars of Gemini the Twins.

Look above and to Gemini's left for a "star" brighter than Castor and Pollux. This is Saturn, now gleaming at magnitude 0.4. Hovering nearby is a ruddy "star" a bit fainter than Castor: the Red Planet, Mars.

This week, Mars courses through the famous Beehive cluster (M44) in the constellation Cancer the Crab — a great treat for anyone who owns binoculars or a telescope. On June 15, as Saturn watches from the wings about 1.2° away, Mars gleams like a brownish-yellow jewel in the midst of the cluster's smattering of stars.

Two nights later, June 17, Mars passes 0.6° north of Saturn. Low-power, rich-field telescopes will show both planets in the same field of view. Mars lies in the foreground, 212 million miles (341 million km) away. Saturn is 917 million miles (1.48 billion km) away, while the Beehive cluster forms the backdrop 580 light-years away. The cluster itself is about 10 light-years across.

Also in the sky (June 13-22):

June 17 — Mars 0.6° north of Saturn

June 18 — Last Quarter Moon, 10:08 a.m. EDT

June 20 — Mercury at greatest eastern elongation (25°). Elongation is the apparent angular separation of an object from the Sun.

June 20 — Mercury 6° south of Pollux

June 21 — Summer solstice 8:26 a.m. EDT

June 22 — Venus 6° south of the Moon in the predawn sky

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