Skywatch May 2016

Skywatch May 2016

May should be a great skywatching month in the islands. Mars will be brighter and closer this May than any time in the last ten years; Jupiter and Saturn also dance in the mid-evening sky. Mercury transits the sun on May 9. Throughout the main Hawaiian Islands, the first of the year’s two “overhead sun” days occur, a phenomenon unique to the tropics.

Planets in May

The Brightest Mars in a Decade

No other planet varies so much in brightness to the naked eye as Mars does. At its most distant, the red planet shines at a feeble 1.8 magnitude, as it comes co around the far side of the sun at a 230 million miles distance from earth. At its closest, Mars a mere 47 million miles away and shines around almost as brilliantly as the planet Jupiter, usually the unrivaled #2 among bright sky dots (Venus, when visible, always wins the brightness contest handily). During the peak nights, May 21-30 2016, Mars will indeed shine brighter than minus 2 magnitude, and will peak on May 22-23 at minus 2.08.

Planets like Mars are generally at their brightest, and at their closest approach to earth, around the time of “opposition.” When a planet is in opposition there is straight line between the sun, earth and Mars, with the earth in the middle of the lineup. The moment of Mars’s opposition occurs at 1:17 a.m. on May 22 Hawai‘i Standard Time (that’s 11:17 on May 22 Universal Time). On that night, Mars will shine at minus 2.08 magnitude, brighter than any star. In fact, you can compare Mars’s brightness that night to Jupiter, which will be moving down the western sky as Mars rises in the southeast. The two planets will be almost identically bright in late May. On May 22, Mars shines at minus 2.08 and Jupiter is just slightly brighter at minus 2.11.

When a planet it is opposition, that means you have the sun on one side of the earth and the planet on the other side. That also means that, as the sun sets in the west, the planet in opposition rises in the east; is high in the south at midnight; and sets at dawn in the west as the sun rises in the east. So not only is Mars at its peak brightness in late May; it’s also overhead all night long.

Interesting, the lineup of the sun, earth and Mars is such that Mar’s closest approach to earth actually not during opposition itself, but instead on May 30, over a week after opposition. Mars will still be brilliant on May 30, at minus 2.03; the planet that day is 46,777,000 miles away.

Mars will in fact be brilliant all month, which makes for a great month of Mars viewing. Please have a look at our accompanying May 2016 star map, which is good for 10 p.m. at the start of May, 9 p.m. in the middle and 8 p.m. at the end of the month. The map shows that Mars is in the claws of Scorpius the Scorpion. Mars is just above the reddish star Antares, which is by brightest star in the Scorpion. Antares in fact looks a lot like Mars, and its name in Greek means ‘the rival of Mars.’ However, this May the planet Mars far outshines the star named for it; Antares always shines at a steady 1.03 magnitude, and Mars shines over a dozen times brighter at Antares.

Mars Through the Month

Early May: Mars rises in the east southeast at 9 p.m.; it is already at minus 1.5 magnitude, already brighter than even the brightest stars. By 9:30 p.m., both Antares and Saturn will have also cleared the horizon. Antares will be 5 degrees below Mars (the width of 2 fingers or so); Saturn, seven degrees to Mars’s left. By 2:15 a.m. Mars will be due south, and almost exactly halfway up in the southern sky.   Mars will be 2/3 of the way down the western sky by the time day breaks.

Mid May: Mars rises in the east southeast about 8 p.m., just as full darkness settles in. Due south at 1 a.m., halfway between the south horizon and the top of the sky; is low in west at daybreak. Now at minus 1.93 magnitude.

May 21-30: Peak days of Mars. Rises in the south southeast at dusk; shines at minus 2.03 on May 21 and peaks on May 22-23 at minus 2.08. By 31st, Mars has returned to minus 2.03 magnitude, still brighter than any star.   Due south around 12:30 a.m.; sets in west right at daybreak.

Also, look for the full moon next to Mars on the night of May 20-21. As the moon and Mars rise that night, Mars will hang like a pendulum below the moon; by the time they set together the next morning, the moon and Mars will be side by side.


Saturn will appear next to Mars all month. It too shows an increase in brightness, from 0.2 magnitude to 0 magnitude at the month goes on. As our May star map shows, Saturn is to the left of Mars and of the Scorpion. Saturn rises at 9:30 p.m. in early May, in the south southwest, coming up about 30 minutes after Mars; it’s due south at 3 am and partway up in the west at break of day. By the end of May Saturn rises at dusk, is due south at 12:45 a.m. and sets at dawn. Throughout the month, you can use Mars as a reference; find that unmissable orange marker first, then look near it for the yellow-white dot of Saturn.


Jupiter has been unrivalled in terms of brightness for our winter and spring evening sky, though Mars is giving is strong competition this May. Throughout the month, it lies just below Leo the Lion. In early May, look for Jupiter 2/3rds of the way up in the east at dusk; it will be high in the south by 9 p.m., and sets in the west at 3 a.m. By the end of the month Jupiter is very high in the southwest at dusk (2/3 of the way between the horizon and the zenith), and sets around 1 a.m.


Venus, while lurking in the eastern sky before dawn for a very long time in the earlier spring, is now entirely gone; it passes behind the sun and we won’t see it again till July.


While Mercury moves between the earth and the sun on May 9, it will get far enough from the sun to be visible just before dawn at the very end of May. Mercury rises at 4:30 a.m. at the end of May and is washed out in the dawn 45 minutes later; even at daybreak it will still be low in the east (ten degrees – the width of a fist at arm’s length). It shines at first magnitude.

Transit of Mercury

A more momentous event for Mercury in May is its passage across the face of the sun on May 9. Mercury moves across the face of the sun about 15 times per century, always in May or November. The most recent transit visible in the islands were on May 2006 and November 1999.

In Hawai‘i Standard Time, this transit begins at 1:15 a.m. on May 9, of course hours before the sun rises and before we can see the transit. On that day, in fact, Mercury will already be 2/3 of its way across the face of the sun when the sun rises at 6:05 a.m. in Honolulu. Using a safe viewing technique, Mercury will appear as a tiny black dot in the middle of the sun’s disk. This transit takes well over seven hours, and we will in fact still have 2.5-plus hours to observe the transit (again with safe viewing equipment) till the transit ends at 8:40 a.m. HST.  Mercury is a tiny speck against the sun’s disk, and you’ll need a telescope with a good solar filter to view it.

Lāhaina Noon

In the tropics the sun passes overhead twice during the year. On these two days, at local noon, the sun will be exactly overhead and an upright object such as a flagpole will have no shadow. This phenomenon only occurs in the tropics; the sun is never overhead in any other part of the planet. The northern reaches of the Hawaiian Islands, such as Midway Island, are north of the tropics and do not experience the overhead sun.

The “overhead sun” date varies depending on how far north or south you are in the tropics. The phenomenon occurs in mid-May in the southern end of the chain. On the Island of Hawai‘i, the overhead sun date occurs on May 18 in both Hilo and Kailua Kona; further north, for example in Līhu‘e on Kaua‘i, the zenith sun day is on May 30.

Here in the islands a term we often use for zenith noon is “Lāhaina Noon.” This is a modern term, selected by Bishop Museum in a 1990 contest held to select a name for the zenith noon phenomenon. The term “Lā haina” means “cruel sun” in Hawaiian, and while the sun in the islands is almost never cruel, it can be pretty intense as it shines directly down from the zenith. (Another phrase that has been suggested for the zenith sun is “kau ka lā i ka lolo,” which translates as “the sun rests on the brains”; but scholars I’ve discussed this with have suggested that this term applies to the sun’s position around noon on any date, and is not specific to the two days a year when the sun is exactly overhead.)

The chart below gives the overhead sun dates for both May and July, the two months each year when all of the main islands experience the phenomenon.

2016 Lāhaina Noon Days and Times

LīhueMay 30 12:35 p.m.July 11 12:42 p.m.
Kāne‘oheMay 27 12:28 p.m.July 15 12:37 p.m.
HonoluluMay 26 12:28 p.m.July 15 12:37 p.m.
KaunakakaiMay 25 12:25 p.m.July 16 12:34 p.m.
Lāna‘i CityMay 23 12:24 p.m.July 18 12:34 p.m.
LahainaMay 24 12:23 p.m.July 18 12:33 p.m.
KahuluiMay 24 12:22 p.m.July 18 12:32 p.m.
HanaMay 23 12:20 p.m.July 18 12:30 p.m.
HiloMay 18 12:16 p.m.July 24 12:27 p.m.
Kailua-KonaMay 18 12:20 p.m.July 24 12:30 p.m.
South Point IslandMay 15 12:19 p.m.July 28 12:28 p.m.

May Star Map and Southern Cross

| Bishop Museum

Like all of our monthly maps, this one is good for 10 p.m. on the first day of the month, 9 p.m. for the middle of the month, and for 8 p.m. at the end.  The map shows the ideal time to find Crux: when the Cross is due south, it is at its highest point, even though still quite low from O‘ahu. Also: by the time Crux due south, the brilliant stars Alpha and Beta Centauri have risen. Find these two first; they are brighter than the stars in Crux and will pop out sooner. Once you’ve found these two brilliant stars, go right, and these two stars will point you to Gacrux, the top star in the Cross.

While the Big Dipper and part of the Scorpion are on this map, we’re missing Orion. You can still catch that great winter constellation low in the west just at dusk for the first ten days of May, but it’s gone by mid-month and gone by star map time.

Moon Phases (Hawaiian Time Zone):

New:  May 6
First Quarter:  May 13
Full:  May 21
Third Quarter:  May 29

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2017-06-05T12:05:54+00:00Monthly Astronomy Article|