A Challenge in Visual Athletics: Hunting the Gegenschein

The gegenschein visible as the bright 'knot' in the zodiacal glow high above the VLT. Image credit: ESO/Y. Beletsky

Looking for something truly elusive? Astronomy has no shortage of the fleeting and ephemeral when it comes to challenges. This week’s challenge will require supremely dark skies and persistence.

We’re talking about the gegenschein, the elusive counter glow lying opposite to the Sun across the plane of the ecliptic. Continue reading “A Challenge in Visual Athletics: Hunting the Gegenschein”

And Mercury Makes Five: See All Naked Eye Planets in the Sky at Once

The waning crescent Moon above Venus and Saturn (dimmer and below Venus) in the dawn twilight on January 6, 2016. The Moon re-visits the grouping in early February. Image credit and copyright: Alan Dyer.

A fine sight greets early risers this week into the month of February, as all five naked eye planets: Mercury, Venus, Mars, Saturn and Jupiter ring the sky from horizon to horizon.

Though not a true planetary alignment as extolled by many websites, this is a great chance to see all five classical planets above the horizon at once… or seven, if you count the waning gibbous Moon and the rising Sun, as the ancients did as part of their geocentric, Earth-entered universe. You can kinda see how they got there, as the very heavens themselves seemed to whorl about the cradle of earthly human affairs. Continue reading “And Mercury Makes Five: See All Naked Eye Planets in the Sky at Once”

Jupiter and the Moon Have a Close Encounter in the Sky February 18, 2013

The January 2013 occultation of Jupiter by the Moon as seen from South America. (Image courtesy of Luis Argerich & Nightscape Photography; used with permission.

The movement of the Moon makes a fascinating study of celestial mechanics. Despite the light pollution it brings to the nighttime sky, we’re fortunate as a species to have a large solitary satellite to give us lessons in “Celestial Mechanics 101″

This weekend, we’ll get to follow that motion as the Moon crosses into the constellation Taurus for a near-pass of the planet Jupiter, and for a very few citizens of our fair world, occults it.

The Moon versus Jupiter during the previous occultation of the planet last month. (Image courtesy of Luis Argerich at Nightscape Photography; used with permission).
The Moon versus Jupiter during the previous occultation of the planet last month. (Image courtesy of Luis Argerich at Nightscape Photography; used with permission).

In astronomy, the term “occultation” simply means that one astronomical body passes in front of another. The term has its hoary roots in astronomy’s ancient past; just like the modern day science of chemistry sprung from the pseudo-science of alchemy, astronomy was once intertwined with the arcane practice of astrology, although the two have long since parted ways. When I use the term “occultation” around my non-space geek friends, (I do have a few!) I never fail to get a funny look, as if I just confirmed every wacky suspicion that they ever had about us backyard astronomers…

But those of us who follow lunar occultations never miss a chance to observe one. You’ll actually get to see the motion of the Moon as it moves against the background planet or star, covering it up abruptly. The Moon actually moves about 12° degrees across the sky per 24 hour period.

The position of the Moon & Jupiter as seen from Tampa (Feb 18th, 7PM EST), Perth, (Feb 18th 11:30UT) & London  (Feb 18th at 19UT). Created by the author using Stellarium.
The position of the Moon & Jupiter as seen from Tampa (Feb 18th, 7PM EST), Perth, (Feb 18th 11:30UT) & London (Feb 18th at 19UT). Created by the author using Stellarium.

On the evening of Monday, February 18th, the 56% illuminated waxing gibbous Moon will occult Jupiter for Tasmania and southern Australia around 12:00 Universal Time (UT). Folks along the same longitude as Australia (i.e., eastern Asia) will see a close pass of the pair. For North America, we’ll see the Moon approach Jupiter and Aldebaran of February 17th (the night of the Virtual Star Party) and the Moon appear past the pair after dusk on the 18th.

Orientation of Jupiter, the Moon & Vesta on the evening of February 18th for North America. (Created by the author in Starry Night).
Orientation of Jupiter, the Moon & Vesta on the evening of February 18th for North America. (Created by the author in Starry Night).

But fret not; you may still be able to spot Jupiter near the Moon on the 18th… in the daytime. Daytime planet-spotting is a fun feat of visual athletics, and the daytime Moon always serves as a fine guide. Jupiter is juuuuuust bright enough to see near the Moon with the unaided eye if you know exactly where to look;

Jupiter captured during a close 2012 pass in the daytime! (Photo by author).
Jupiter captured during a close 2012 pass in the daytime! (Photo by author).

To see a planet in the daytime, you’ll need a clear, blue sky. One trick we’ve used is to take an empty paper towel tube and employ it as a “1x finder” to help find our target… binoculars may also help! To date, we’ve seen Venus, Jupiter, Sirius & Mars near favorable opposition all in the daylight… Mercury and Vega should also be possible under rare and favorable conditions.

This week’s occultation of Jupiter is the 3rd and final in a series that started in December of last year. The Moon won’t occult a planet again until an occultation of Venus on September 8th later this year, and won’t occult Jupiter again until July 9th, 2016. We’re also in the midst of a long series of occultations of the bright star Spica (Alpha Virginis) in 2013, as the Moon occults it once every lunation from somewhere in the world. Four major stars brighter than +1st magnitude lie along the Moon’s path near the ecliptic; Spica, Aldebaran, Regulus, and Antares which we caught an occultation of in 2009;

Also of note: we’re approaching a “plane-crossing” of the Jovian moons next year. This means that we’ll start seeing Callisto casting shadows on the Jovian cloud tops this summer on July 20th, and it will continue until July 21st, 2016. The orbits of the Jovian moons appear edge-on to us about every five years, and never really deviate a large amount. Callisto is the only moon that can “miss” casting a shadow on the disk of Jupiter in its passage.  The actual plane crossing as seen from the Earth occurs in November 2014. Jupiter reaches solar conjunction this year on June 19th and doesn’t come back into opposition until early next year on January 5th. 2013 is an “opposition-less” year for Jupiter, which occurs on average once per every 11-12 years. (One Jovian orbit equals 11.8 Earth years).

The Moon plus Jupiter during last month's close conjunction. (Photo by author).
The Moon plus Jupiter during last month’s close conjunction. (Photo by author).

But wait, there’s more… the Moon will also occult +7.7th magnitude 4 Vesta on February 18th at~21:00 UT. This occultation occurs across South America and the southern Atlantic Ocean. It would be fun to catch its ingress behind the dark limb of the Moon, and we bet that a precisely timed video might just show evidence for Vesta’s tiny angular diameter as it winks out. For North American observers, Vesta will sit just off the northern limb of the Moon… if you have never seen it, now is a great time to try!

Finally, we realized that also in the field with 4 Vesta is an explorer that just departed its environs, NASA’s Dawn spacecraft. Although unobservable from Earth, we thought that it would be an interesting exercise to see if it gets occulted by the Moon as well this week, and in fact it does, for a very tiny slice of the planet;

The occultation of the Dawn spacecraft as seen from Earth. Created by the author using Occult 4.0.
The occultation of the Dawn spacecraft as seen from Earth. Created by the author using Occult 4.0.

Hey, calculating astronomical oddities is what we do for fun… be sure to post those pics of Jupiter, the Moon and more up to our Universe Today Flickr page & enjoy the celestial show worldwide!

See more of Luis Argerich’s astrophotography at Nightscape Photography.

Graphics created by author using Stellarium, Starry Night and Occult 4.0 software.

Mercury’s False Moon: The Mercury/Mars Planetary Conjunction this Weekend

Mercury and Mars on February 8, 2013. See how close they'll be? Image credit: Stellarium.


The history of astronomy is littered with astronomical objects in the solar system that have fallen to the wayside. These include fleeting sightings of Venusian moons, inter-mercurial planets, and even secondary moons of the Earth.

While none of these observations ever amounted to true discoveries, this weekend gives observers and astrophotographers a unique chance to “mimic” a spurious discovery that has dotted astronomical lore: a visual “pseudo-moon” for the planet Mercury. This “moon illusion” will occur on February 8, 2013 during the closest conjunction of two naked eye planets in 2013. February offers a chance to see the fleeting Mercury in the sky, and this conjunction with Mars will provide the opportunity to see how Mercury would look in the night sky if it had a moon!

Mercury has been suspected of having moons before. On March 29th 1974, the Mariner 10 spacecraft became the first mission to image the innermost world up close. Mariner 10 mapped 40-45% of Mercury on 3 successive passes, revealing a pock-marked world not that different than our own Moon. But Mariner 10 also detected something more: brief anomalies in the ultra-violet spectrum suggestive of a moon with a 3 day period. For a very brief time, Mercury was thought to have a moon of its own, and NASA nearly made a press release to this effect. The spectroscopic binary 31 Crateris is now suspect in the anomalous readings. Still, the Mariner 10 observation made researchers realize the observations in the extreme UV were possible over interstellar distances.

The planet Mercury as seen by NASA's Messenger spacecraft (Credit: NASA/JPL).
The planet Mercury as seen by NASA's MESSENGER spacecraft (Credit: NASA/JHUAPL).

Today, NASA has a permanent emissary orbiting Mercury with its MESSENGER spacecraft. MESSENGER first entered orbit around Mercury on March 18th, 2011 after a series of trajectory changing flybys. MESSENGER has filled in the map of the remainder of Mercury’s surface, with no signs of the anomalous “moon.” Interestingly, MESSENGER was also on the lookout for “Vulcanoids” (tiny asteroids interior to Mercury’s orbit; sorry, Mr. Spock) while enroute to its final orbital insertion. NASA even released an April Fool’s Day prank of a fake “discovery” of a Mercurial moon dubbed Caduceus in 2012.

But MESSENGER has made some fascinating true to life discoveries, such as sampling Mercury’s tenuous exosphere & the possibility of ice at its permanently shadowed poles. Lots of new features have been mapped and named on Mercury, following the convention of naming features after famous deceased artists, musicians and authors set forth by the International Astronomical Union. It’s amazing to think that we had no detailed views at the entire surface of Mercury until the 1970’s, although some ground-based professional observatories and even skilled amateurs are now doing just that.

Fast forward to this weekend. Mercury is just beginning its first apparition of six in 2013 this week and is currently visible low in the dusk sky after sunset to the west. Mercury reaches greatest eastern elongation on February 16th at 18.1° from the Sun. Interestingly, that’s very close to the shortest elongation that can occur. Mercury’s orbit is eccentric enough that greatest elongation as seen from the Earth can vary from 17.9° to 27.8°. This month’s elongation happens within only 5 hours of Mercury reaching perihelion at 46 million kilometers from the Sun. This means that Mercury won’t peak above the dusk horizon for mid-northern latitude observers quite as high as it will during the next evening apparition of the planet in June.

Mercury Mars
caption =”Looking west 30 minutes after sunset on Feb. 8th from latitude 30° north.


This appearance of Mercury does, however, have some things going for it. First off, the ecliptic sits at a favorable viewing angle, roughly perpendicular to the western horizon at dusk for mid- to high northern latitude observers. This gives Mercury a bit of a “boost” out of the weeds. Secondly, Mercury is a full magnitude (2.512 times) brighter when it reaches maximum elongation near perihelion than aphelion, such as its next appearance in the dawn sky on March 31st of this year. Mercury will reach magnitude -0.5, versus +0.5 in late March.

To see Mercury, find a site with a western horizon free of ground clutter and start sweeping the horizon with binoculars about 15 minutes after local sunset. See a reddish dot just above Mercury? That’s the planet Mars, shining about 7 times fainter than -1.0 magnitude Mercury at magnitude +1.2. Mercury is fast approaching a conjunction with Mars; the two will be only 15’ apart (half the average width of a Full Moon) on the evening of February 8th at 17:00 Universal Time!

If you ever wondered how Mercury would appear with a moon, now is a good time to take a look! Again, binoculars are the best optical tool for the job. Can you see both with the naked eye? Can you place both in the same low power field of view with a telescope? You’ll only have a 15-30 minute window (depending on latitude) to snare the pairing before they follow the setting Sun below the horizon. Photographing the pair will be tricky, though not impossible, as they present a very low contrast against the bright background twilight sky.

caption =”Mercury (lower center) & Mars (upper center) imaged by Mike Weasner on February 5th.


Don’t expect to see detail on Mercury or Mars telescopically; Mercury only appears 5.8” across on the 8th, while Mars is 4” in apparent size. Mars disappears from view later this month to reach solar conjunction on April 18th 2013. The waxing crescent Moon just 1 day after New joins the pair on the evenings of February 10th and 11th.

Now for the “Wow” factor of what you’re seeing. The conjunction of Mars and Mercury only appears close; in reality, they are over 180 million kilometers apart. Mercury is 1.15 Astronomical Units (A.U.s)/178 million kilometers from us on February 8th, while Mars is nearly at its farthest from us at 2.31 A.U.s/358 million kilometers distant. It’s splendid to think that with Curiosity and friends operating on Mars and Messenger orbiting Mercury, we now have permanent robotic “eyes” on and around both!

Credits: Simulation created by the author using Starry Night.

Mercury & Mars courtesy of Mike Weasner and the Cassiopeia Observatory. Used with permission.