From Astronomy Picture of the Day

From Robert C. Victor

Jeff,

This is dated material. Some of the events happen this week. Please share with others who may be interested. Thank you!

Skywatchers in midwestern U.S.,

On most of the last several mornings, I have taken my 6-inch Orion Dobsonian reflector outdoors during the couple of hours before sunrise, and used a 6-mm eyepiece (providing a magnification of 200-power) to observe Mars and Saturn. The south polar cap of Mars was immediately obvious!

The Martian southern hemisphere spring equinox occurred on May 22, so the polar cap is still near its maximum extent. I also noticed some dark markings on the disk, near the equator and southern latitudes, and a bright area near the following or morning (celestial east, Martian west) limb, which may have been morning frost or morning clouds. And at the northern limb, I noticed a very narrow bright area that could have been the edge of the northern polar hood of clouds. The north pole itself is tipped away from Earth by 15 degrees.

Keep in mind that as spring progresses in the Martian southern hemisphere, the southern polar cap will shrink rapidly, so if you wait until Mars becomes conveniently visible in the evening, a much smaller polar cap will remain. Opposition and closest approach of Mars will occur in the last week of July, but don’t wait until then!

In coming days, two of the most striking markings on Mars (in addition to the South Polar Cap) will be in excellent position as seen from the eastern U.S. in the hours before dawn, when Mars is highest in the southern sky.

The markings are (1) Syrtis Major, which displays a dark triangular shape resembling a northward-pointing India, centered just north of the Martian equator. Also at the same Martian longitude (290 degrees) is (2) Hellas Basin, which often appears very bright because of haze or surface frost deposit. Hellas is centered near lat. 42 degrees south and so appears between Syrtis Major and the South Polar Cap.

Currently, any particular Martian surface feature reaches the central meridian of Mars a little over 38 minutes later each day. So if you observe Mars at the same time each morning, a feature will first appear near the sunset terminator, near the celestial west limb of Mars where the solar illumination cuts off. (You’ll notice in early June that Mars appears not round, but in gibbous phase.) Each successive day at the same time, the Martian feature will move farther back from the sunset terminator, eventually passing near the center of the disk, and continuing on toward the morning (celestial western) limb. In each 24 hours, Mars makes less than one complete rotation.

Here are times (in CDT) when the longitude of the central meridian of Mars is equal to 290 degrees. At these times, Syrtis Major will appear a little north of the center of the Martian disk, and Hellas will appear well south of the disk center. Even if you observe up to three hours away from these times, the east-west foreshortening of these features will be less than 50 percent, but you’ll not want to observe too many hours before Mars reaches your local meridian — it’ll be too low for good seeing — or much after Mars crosses your local meridian, because you’d be viewing in the daytime.

Longitude of central meridian of Mars = 290 degrees;
Syrtis Major and Hellas transiting central meridian.

Tuesday      June  5   2:26 a.m. CDT
Wednesday June  6   3:04 a.m.
Thursday     June  7   3:43 a.m
Friday          June  8   4:21 a.m.
Saturday      June  9   4:59 a.m.
Sunday       June 10   5:38 a.m. (near sunrise, but observe earlier,
as these features approach the central meridian)

Monday    June 11   6:16 a.m.  ” ”
Tuesday   June 12   6:54 a.m.  ” ”

RCV