Maximum Mercury, and the Old Moon’s Tour of Pre-Dawn Planets gives us Dark Night Delights!

Star Walk
7 min readFeb 25, 2019


See the “Demon” Star Brighten

The “Demon Star”, more formally known as Algol or Alpha Persei, is a star that is easy to see using unaided eyes under suburban and rural skies. Algol’s visual brightness dims noticeably for about 10 hours once every 2 days, 20 hours, and 49 minutes — like clockwork. Located in Perseus (the Hero), this star is among the most accessible variable stars for beginners because the amount it varies by is large and it remains bright enough to see without optical aid when it dims.

Contrary to the gory colours that might be inspired by its nickname (it represents one glowing eye in the severed head of Medusa the Gorgon), Algol is actually a hot, white star located 92 light-years from Earth. Algol’s regular dimming happens because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star — an arrangement that is called an eclipsing binary star system. The dimming periods can fall at any time of the day or night. This week, the timing of one cycle makes watching the return to brightness a convenient project for evening observers.

This Friday, March 1 at 8:04 pm EST, Algol will be at its minimum brightness of magnitude 3.4. At that time, the star will sit a bit more than halfway up the western sky — 2.3 fist diameters directly below the bright yellow star Capella. Over the following five hours, Algol will descend into the west while it steadily brightens. By 1 am EST, it will be low over the northwestern horizon and will have brightened to its usual magnitude of 2.1.

The Moon and Planets

Great news for astronomers! This week, the moon will be completely out of the evening sky, leaving it nice and dark for chasing galaxies, star clusters, and nebulas. Instead, the waning moon will be seen in the southeastern pre-dawn sky while it slides towards next week’s new moon phase. The moon will also make its monthly visit to the bright pre-dawn planets this week.

On Monday morning before sunrise, the moon will sit among the stars of Libra (the Scales). On Tuesday morning, the moon will reach its last quarter phase — when it will be illuminated from the side by the pre-dawn sun and will appear with a “half-moon” shape. At this point in the moon’s monthly cycle, it rises after midnight and lingers into the daytime morning sky.

In the southeastern pre-dawn sky on Wednesday, the waning last quarter moon will sit 2 finger widths to the upper right of the very bright planet Jupiter. Both objects will fit into the field of view of binoculars and telescopes at low magnification. Observers in western North America and the Pacific region will see the moon when it is even closer to Jupiter.

On Friday morning, the waning crescent moon will be located three finger widths to the upper right of yellowish Saturn. Both objects will fit into the field of view of binoculars. Later that same day, observers in most of Micronesia, northern Polynesia (except Hawaii), Central America, and Southern North America can see the moon cross in front of (or occult) Saturn in daylight. The following morning, between about 5 am local time and dawn, the slim crescent moon will be positioned 4.5 finger widths to the right of bright Venus. The pair will make a lovely photo opportunity when composed with a foreground landscape.

(On Tuesday evening, Mercury will swing widest from the sun, allowing it to remain visible in the darkening sky after sunset. The best times to look for the planet fall between about 6:15 and 7:15 pm local time. After Tuesday, Mercury will slowly descend towards the sun.)

This week’s evenings are your absolute best chances to see elusive Mercury during 2019, if you live in the Northern Hemisphere! On Tuesday evening, Mercury will reach its widest separation from the Sun. Because Mercury will be sitting above a nearly vertical evening ecliptic, the sky will begin to darken while Mercury is still well above the western horizon — revealing the planet clearly.

The optimal viewing times fall between 6:15 and 7:15 pm local time. If you view Mercury in your small telescope, the planet will exhibit a waning half-illuminated disk. Find a viewing spot where the western horizon is low and free of foreground obstructions. Once the sun has fully set, sweep the sky with binoculars — or your own sharp eyeballs — looking for a medium bright, unmoving point of light.

(As shown here at 7 pm local time on Tuesday, early evening this week offers the chance to find three planets — Mercury and Mars will be easy, while Uranus will require binoculars and star chart.)

The other easy evening planet to see this week will be Mars. When the sky begins to darken, Mars will appear as a medium-bright, reddish pinpoint of light about halfway up the western sky. The Red Planet will set at about 11:15 pm local time. Mars has been slowly shrinking in size and brightness as we increase our distance from it little-by-little.

Mars is still positioned less than a fist’s diameter above the much dimmer, blue-green planet Uranus. During this week, Mars will continue to climb away from Uranus. In the meantime, the distant ice giant planet can be identified by aiming binoculars about 1.6 finger widths above the modestly bright star named Torcular (or Omega Piscium). Look for Uranus right after dark — this week the planet will set at around 10:30 pm local time.

If you are willing to venture outside on a clear morning this week, you can enjoy the spectacle of the moon plus three bright planets — Jupiter, Saturn, and Venus, appearing low in the eastern morning sky.

Bright Jupiter will rise first, at about 3 am local time. By dawn, it will be a beacon in the southern sky. Yellowish Saturn, which is twice as far away as Jupiter, is correspondingly dimmer. The ringed planet will rise at about 4:30 am local time and will be lost in the twilight by 7 am. Our sister planet Venus is only one-fifth as far from Earth as Jupiter. Venus’ blazing brilliance will grace the southeastern dawn sky after 5 am local time, and remain in view until sunrise. In a telescope, Venus will exhibit a gibbous (more than half-illuminated) phase.

Dark Night Delights

With the moon out of the evening sky this week and next, it’s time to grab your binoculars and telescopes and explore the darker sky for treasures. Deep sky objects are astronomical sights that are beyond our solar system. Here are some suggestions for viewing.

Double stars and multiple stars are generally positioned less than a few hundred light-years from the sun, close enough for us to see them as individual stars. Among the best and easiest ones for late winter are Sigma Orionis (a finger’s width below Orion’s leftmost belt star), many of the stars forming Taurus’ triangular face, the star Almach (which is 2 fist diameters to the left of Cassiopeia), and Castor (the higher of the Gemini twins).

Open star clusters are concentrations ranging from tens to hundreds of stars. This class of deep sky object is farther away than double stars — from a few hundred to a thousand light years distant. The brightest and easiest cluster to see in late February is the Pleiades Cluster (Messier 45). This cluster is composed of a compact, bright group of six stars three times the diameter of the full moon located high in the western evening sky, about 3.5 fist’s diameters to the right of Orion’s Belt.

(This image of the Pleiades Cluster, or Messier 45, was taken by Stuart Norman of Toronto on October 19, 2017 from the Blue Mountains, Ontario. This large object, three times the moon’s diameter, is best viewed in binoculars or a telescope at low magnification. The blue nebulosity is foreground dust scattering the bright starlight.)

Another big cluster surrounds the bright star Mirfak in Perseus (the Hero). A gorgeous, bright pair of clusters called The Double Cluster is located midway between Mirfak and Cassiopeia. Sweeping the sky with binoculars, you can find more clusters in Cassiopeia (the Queen) and Auriga (the Charioteer).

Nebulas are collections of glowing, cold interstellar hydrogen gas distributed along our galaxy’s spiral arms. Nebulas are the birthplace of star clusters, so they have to be larger in diameter than clusters. The best nebula in the winter sky is the Orion Nebula (Messier 42). It’s the central patch of light in Orion’s sword. Another, dimmer nebula named the Rosette Nebula sits between and slightly above the imaginary line connecting the star Procyon to Orion’s Belt. It’s about 5,000 light-years away!

Once we’ve left our galaxy behind, we have to look a long way away for our final type of deep sky object — the galaxies. The closest and brightest galaxy in the late winter night sky is the Andromeda Galaxy (Messier 31). Its light has been journeying to our eyes for 2.5 million years! The Andromeda Galaxy sits in the northwestern evening sky, 1.5 fist diameters to the lower left of Cassiopeia.

Over in the eastern evening sky, the evening constellation Leo (the Lion) hosts a tremendous number of distant, dim galaxies, too. But we’ll save them for another day. Good luck with your hunt!

Evening Zodiacal Light

For about half an hour after dusk between today and the new moon on March 6, look west-southwest for a broad wedge of faint light rising from the horizon and centered on the ecliptic. This is the zodiacal light — reflected sunlight from interplanetary particles of matter concentrated in the plane of the solar system. The glow will be centred on the horizon directly below Mars. Try to observe from a location without light pollution, and don’t confuse the zodiacal light with the brighter Milky Way to the northwest.

(Zodiacal light. Image Credit:ESO (Yuri Beletsky))

Astronomy Skylights for the week of February 24th, 2019 by Chris Vaughan.

Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!



Star Walk

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