September/October 2017 – Vol. 30 No. 1

Celestial Highlights for April and May 2016

Posted: Friday, April 8th, 2016

by Robert C. Victor. Twilight sky maps by Robert D. Miller. Planet rising and setting graphs by Jeffrey L. Hunt.

Jupiter, and Sirius until it departs, continue to dominate evening sky (April and May). This year’s best evening appearance of Mercury in mid-April precedes its transit across the Sun on May 9. Mars kindles to its brightest and closest since 2005. Mars-Saturn-Antares triangle, prominent in morning, rises earlier in evening as weeks pass. “Blue Moon” and Red Mars team up on May 21. Don’t miss this spring’s chances to get close-up telescopic views of other planets!

Apr. 7–New Moon 4:24 a.m. PDT. Moon at perigee 11 a.m. Large tides!

Apr. 8–Young crescent Moon, age 39 hours, easy to see in twilight. Look for Mercury to Moon’s lower right, and earthshine on dark side of Moon.

Apr. 10–Moon occults Aldebaran in daytime (use telescope). In evening, find this star and Hyades cluster closely lower right of Moon, a spectacular sight for binoculars!

Apr. 12–Spica at opposition, visible all night.

Apr. 13–Moon at First Quarter, half full in afternoon and evening sky. Today and for a day or two before and after, install a single polarizing filter in your telescope’s low-power eyepiece and rotate the eyepiece to darken the sky and improve contrast of Moon against the blue daytime sky.

Apr. 16-23–Before dawn [April / May] Mars-Saturn reach minimum distance apart, 7.2°. Stopping short a few degrees NNW of Antares, Mars begins to retrograde on Apr. 17. The red planet will move nearly 16° west by June 29; watch it pass 2nd-mag Delta in middle of Scorpion’s head in mid-May, and again in early August. Mars will pass between Saturn and Antares in an eye-catching evening alignment on Aug. 24.

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Apr. 17–Moon near bright Jupiter at dusk. On Apr. 17 and 18, Mercury stands near greatest elongation, reaching its greatest altitude at dusk for 2016, but begins to fade rapidly later this week.

Apr. 20, 21–Moon near Spica most of night. Full Moon Apr. 21, 10:24 p.m. PDT.

Apr. 24, 25–Four hours after sunset until dawn, spectacular gathering of Moon, Mars, Saturn, and Antares. See below.

May 7–At dusk, look for first young Moon, age 31-32 hours, very low in WNW. Aldebaran, eye of Taurus the Bull, just to upper left, beautiful in binoculars! On May 8, Moon will be higher, to upper left of Aldebaran.

May 9–Transit of Mercury. Details below.

May 14, 15–Moon near Jupiter. Saturday May 14 is ASTRONOMY DAY. https://www.astroleague.org/al/astroday/astrodayform.html

Find a club, planetarium, or observatory and participate in their activities.

May 21–Full “Blue Moon” and red Mars hang together tonight. In spring 2016, we have four Full Moons: On Mar. 23, Apr. 21, May 21, and Jun. 20. The third Full Moon of four within the same astronomical season is called a “Blue Moon”. https://en.wikipedia.org/wiki/Blue_moon

So tonight’s Full Moon, the third of four this spring, is a “Blue Moon”.

For illustrations of these and other events in May, you are encouraged to download and reprint the free May 2016 Sky Calendar and evening sky map available at www.abramsplanetarium.org/skycalendar/

[NOTE TO EDITOR: This link will be updated by mid-April, to provide the May 2016 Sky Calendar and evening sky map.]

A graph of the rising and setting times of the planets for southern California may be viewed here.


In mid-April at dusk, Jupiter shines bright and steady, well up in southeast, while next in brilliance, the blue-white “Dog Star” Sirius twinkles in the southwest. Catch Mercury before it reaches its high point low in WNW April 17 and 18, and it will still outshine golden Arcturus in ENE to E. This innermost planet of our solar system passes greatest elongation on those dates, when it appears farthest from the Sun, 20° this time around.

Mercury will get farther from the Sun on other occasions this year, but this time the planet is almost directly above the Sun, which is 9° below the horizon at the time of our evening charts.  This arrangement results in the best apparition of Mercury in the evening sky of this year. But don’t wait! As it circles the Sun and comes around to the near side of its orbit, Mercury shows less of its sunlit side. Moreover, features large and small on its rough, rocky surface cast shadows, causing the planet to fade rapidly after greatest elongation. Mercury shines at mag. –1 on Apr. 7-8, mag. 0 on Apr. 17, fading to mag. +1 on Apr. 22-23, and +2 on Apr. 27.

If proper equipment is used and precautions taken to avoid eye damage, your classes will be able to observe Mercury in silhouette against the disk of the Sun on Monday morning, May 9. During this transit (links below) across the Sun, the disk of Mercury will appear only 0.2 arcminute across, only 1/158 of the Sun’s 31.7’ (just over half a degree) diameter. Therefore, the tiny dot cannot be detected by simply looking through a solar filter without magnification. Instead, use a telescope suitably protected by a certified safe solar filter securely installed at the front end of the telescope, before sunlight enters the optical system. Use a magnification of at least 50-power. Or, use your telescope to project an image of the Sun on a screen or a piece of white cardboard. Whichever method is used, be sure to remove the finder scope so that no curious student will be tempted to look through it at the Sun.

These same techniques can be used to observe a partial solar eclipse, or sunspots. When using the projection method without a solar filter, if your telescope is greater than about 3 inches in diameter, put a secure cap with a hole in it over the front end to reduce the size of the aperture.

From California on May 9, the transit will already be underway at sunrise. From Palm Springs, sunrise occurs at 5:49 a.m. PDT, and the planet passes closest to the center of the solar disk at 7:58:31 a.m., with the Sun 25° up in the east. Mercury will then be slightly more than one-third of the way from the Sun’s center toward its SSE limb. For the next 3-2/3 hours, Mercury slowly creeps toward the SW limb of the solar disk, until at 11:39:06 a.m., when the leading edge of Mercury meets the edge of the disk. Egress lasts 3.2 minutes, until 11:42:18 a.m., when the disk of Mercury moves completely off the solar disk. Times for other locations in California differ only a few seconds from these times.

After May 9, the next transit of Mercury will occur on Nov. 11, 2019, also a morning event from California. The transits in Nov. 2032 and Nov. 2039 occur at night for Californians, so won’t be seen here. After 2019, for the next transit of Mercury visible in our state, we’ll have to wait until May 7, 2049.

Links to transit of Mercury and safe solar viewing:

(See especially these sections in the above: Projection with binoculars or a telescope; Telescope viewing.)

(Check www.skyandtelescope.com for an update for the transit of May 9, 2016.)


After Mercury fades away into the bright western twilight in late April, there are still several bright stars remaining in the western sky. Each evening within an hour after sunset, keep track of the stars Rigel, Aldebaran, Sirius, and Betelgeuse. By late in May, they’ll all be gone, while Mars, Saturn, and Antares will rise into the southeastern sky at dusk. In June, the “Spring Arch” of Procyon, Pollux (with Castor, not shown), and Capella will sink into the western twilight. See last’s month’s article in California Classroom Science for a desktop activity using orbit charts and a table of planet positions to explain all these changes.

Bright Jupiter crosses very high in the south in the course of May. To its west is the heart of Leo, the star Regulus. At mag. +1.4 it is the faintest of the 21 stars of first magnitude or brighter (15 or 16 are visible from California).

Rising into view in the southeastern sky in midtwilight during the latter half of May are brilliant reddish Mars, with Saturn following to its lower left, and the reddish twinkling star Antares “Rival of Mars”, below Mars and right of Saturn.

In April and early May, you can stay up late into the evening to view the pretty triangle of Mars-Saturn-Antares. For example, on the night of April 24, four hours after sunset, see this triangle below and lower right of the waning gibbous Moon, while Jupiter shines high in the southwest. On April 25, at the same late hour, you’ll find the Moon low in ESE, to the lower left of the triangle.

At Moon’s next time around, on May 21, the Full Moon and Mars will rise nearly together, right around sunset. Mars will be at opposition that night, and above the horizon all night long, dusk until dawn. At predicted peak brilliance, mag. –2.1, Mars, except for its rusty color, will appear a close match for Jupiter! Closest approach of Mars, 47 million miles, occurs on May 30, the best since Oct.-Nov. 2005.

Jupiter with its cloud belts and four bright moons discovered by Galileo, and Saturn with its rings, are impressive showpieces for even small telescopes! But Mars is usually not, except within a couple of months of its oppositions, when you can get glimpses of surface features. At this year’s opposition, Mars is low in the sky, so it is best viewed when it’s highest, as it passes due south.

Observe Mars: See surface features on another world, its rotation, and seasonal changes!

Links for telescopic observation of Mars:

By coincidence, of the very night of its opposition, May 21-22, the most prominent visual feature of Mars, a triangular dark marking called Syrtis Major, will appear at the center of the Martian disk when Mars is highest in the southern skies of California. In fact, from Palm Springs, Mars passes its high point, due south and 35° up, at 12:43 a.m. PDT on May 22, while Syrtis Major straddles the central meridian of Mars! Because Mars’ day is slightly longer than Earth’s, around opposition we’ll see the same face of Mars about 36 minutes later on each successive night, thus, on May 16 at 9:43 p.m. (Mars 16° up for Palm Springs); May 17 at 10:19 p.m. (22° up); May 18 at 10:55 p.m. (27° up); May 19 at 11:31 p.m. (31° up); May 21 at 12:07 a.m. (34° up); May 22 at 12:43 a.m. (35° up); May 23 at 1:19 a.m. (34° up); May 24 at 1:55 a.m. (31° up); May 25 at 2:31 a.m. (27° up); May 26 at 3:07 a.m. (22° up); and May 27 at 3:43 a.m. (16° up). Note at the first and last dates of this series, Mars is quite low, and the seeing is not likely to be good when Syrtis Major is on the Martian central meridian.

You can also observe Mars at the same time on consecutive nights. Then you’ll see the planet completing 9° less than a full rotation from one night to the next, and you’ll get a slowly changing view of all longitudes of the planet in about six weeks. After 41 days, you’ll see the original features return at the same time of night. Better yet, look nightly when Mars reaches its highest point in the south. The longitude of the central meridian of Mars will change about 10° daily, and after about five weeks, you’ll see the same features return when Mars is highest in the sky. In the best months for viewing, near opposition and closest approach, Mars reaches its high point about 4 or 5 minutes earlier each night.

So Californians will have excellent views of Syrtis Major near Mars’ central meridian around these dates and times when the planet appears due south in Palm Springs:

  • April 16 at 3:35 a.m. PDT
  • May 22 at 12:43 a.m.
  • June 26 at 9:43 p.m.
  • July 31 at 7:52 p.m.

Extend the table to adjacent days, as was done for May 22, above.

Look south of Syrtis Major for the Hellas Basin, of lighter color, possibly brightened even more by frost on its floor. The South Pole of Mars is even farther south (of course), hidden beyond the southern limb of Mars until late September, but the surrounding South Polar Cap (or possibly an overlying hood of clouds) extends well onto our side. The South Polar Hood, if present, will clear, revealing the South Polar Cap of frozen CO2 and H2O near its maximum extent. To determine directions on the Martian disk, nudge your telescope slightly toward Deneb in the sky, and the sky north of Mars will come into view. (The planet’s axis points several degrees north of Deneb.)

The North Pole of Mars is tipped less than 10° toward us until late May, so whatever remains of the surrounding polar cap will appear greatly foreshortened. The end of Martian northern summer, the season of greatest sublimation of the northern polar frozen CO2, is marked by the planet’s autumnal equinox on July 4. Don’t expect to see much of the remnant North Polar Cap, but you might catch the development of an overlying North Polar Hood.

Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs.

Robert D. Miller, who provided the twilight charts and the planet orbit charts, did graduate work in Planetarium Science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

Dr. Jeffrey L. Hunt, a retired planetarium director now living in the Chicago area, has taught astronomy and sky watching to all ages. He studied astronomy education at Abrams Planetarium at Michigan State University. Jeff writes an astronomy blog at jeffreylhunt.wordpress.com and can be followed on Twitter at @jeff_hunt.

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Written by Robert Victor

Robert Victor

Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs, CA. Robert is a member of CSTA.

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