Skywatch May 2017

Skywatch May 2017

Lāhaina Noon returns!

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 flag pole will have no shadow. This phenomenon only occurs in the tropics; the sun is never overhead in any other part of the planet. As the only tropical state, Hawai‘i is the only state to experience the overhead sun. All of the main Hawaiian Islands are in the tropics and thus experience overhead sun; the northern reaches of the chain, such as Midway Island, are north of the tropics and do not experience Lahaina Noon.

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 island chain. Thus, 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 31.

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.

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.

2017 Lāhaina Noon days and times

LīhueMay 31 12:35 p.m.July 12 12:42 p.m.
Kāne‘oheMay 27 12:28 p.m.July 15 12:37 p.m.
Honolulu May 26 12:28 p.m.July 16 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.
Hana May 23 12:20 p.m.July 18 12:30 p.m.
Hilo May 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 Island of Hawai‘iMay 14 12:19 p.m.July 27 12:28 p.m.


Planets in May

Jupiter still king of the evening

The king of the planets is a blazing light that you have probably noticed already in the April sky. Early in May, look for Jupiter about 1/3 of the way the eastern sky at dusk, shining at a blazing minus 2.4 magnitude; Jupiter will be high in the south at 11 p.m. and will set in the west by 4:45 a.m.  By the end of the month, Jupiter is very high in the east at dusk, is due south at 8:45 p.m. and sets at about 2:45 a.m. By the end of the month Jupiter dims slightly to minus 2.24 magnitude, but is still brighter than any star; it remains more than 20 times brighter than the bright blue-white star Spica, which is just under a palm’s width to Jupiter’s left.

Look for the waxing moon next to Jupiter on the nights of May 6 and 7.

Venus lights up the hours before dawn

Venus rises in the east at 4 a.m. in early May and is about 20 degrees up in the east (the width of two palms at arm’ length) at daybreak. In early May Venus shines at an fiery minus 4.5, six times brighter than Jupiter. For the first week or so of May, if you have a flat horizon both to the west and to the east, you can compare these two brightest sky dots; Jupiter will be setting in the west around 4:30 a.m. as Venus rises in the east.

By the end of the month, Venus rises by 3:30 a.m. and is about 1/3 of the way up in the west at daybreak.

In the hours before dawn on May 22, look for the beautiful sight of a very slender waning crescent moon just below Jupiter.

Saturn also brightens the evening hours

Saturn rises in the east by 10:30 p.m. in early May, is due south about 4 a.m., and is high in the west as day breaks.  By the end of the month, Saturn is up at 8:30 p.m., due south at 1:30 a.m., and low in the west at dawn.  Saturn shines steadily at 0 magnitude, as bright as a good bright star, and has a white-yellow color. A good guide for Saturday this year is Scorpius the Scorpion, known here as Maui’s Fishhook; if you find that big bold constellation first, Saturn is to the left of the Scorpion.  Saturn is in fact about 20 degrees (the width of two palms) to the east of the bright, ruddy star Antares, the heart of the Scorpion. Look for the just-past-full moon next to Saturn on May 12 and 13.

Last month for Mars

Mars has clung on in our western sky at dusk for months; this month, it comes to an end. In May, look for Mars about ten degrees above the western horizon (the width of a palm) at dusk; through the month, it sets around 8:30 p.m.  Mars shine faintly at 1.6, not much brighter than, say, the stars of the Big Dipper.  By mid-June we’ll lose the red planet.

Mercury returns to early morning

For the last half of May, look for Mercury about 5 degrees (less than the width of three fingers) about the east horizon, between 4:45 a.m. and dawn. The easiest morning to spot it will be on May 23, where Mercury will be a mere five degrees to the left of a very slender crescent moon.



Other April events

Eta Aquarid meteor shower

The peak of this annual shower will be the night of May 6-7, 2017.  The portion of Aquarius where the stars appear to come from rises above the horizon around 3:45 a.m. This is a light shower, with 10 to 30 meteors or so per hour. One claim to fame: this shower is one of two showers caused by debris from Halley’s Comet, along with the Orionids in October. We have a waxing gibbous moon on that night, which will give off a lot of light; there will be a ‘sweet spot’ early on May 7 between moonset at 4 a.m. and the first light of day around 4:45 a.m. to look for shooting stars. As always, meteor showers are always better from midnight to dawn.

Goodbye, Winter Stars

Every year, May is the month when the great constellation of winter disappear into the light dusk, one after another. On May 1, the Pleiades are still visible low in the west at dusk; within  few days, they are lost in the light of the setting sun. In early May, Orion is still striking, low in the west, at first darkness; by the middle of the month, Orion’s lost in the light of dusk as well. By the end of May, the brightest star, Sirius, and the constellation Auriga are also gone. Among the great winter constellations, only Gemini and Canis Minor shine on into early June, before they too plunge into the light of the sun.

By the way: while we think of Orion and these other constellations as winter stars, they are actually visible most of the year; for example, while Orion departs from the spring evening sky by May 15, it pops up in the predawn sky each year by July 31, after being absent for only ten weeks.


May Star map

The May star map is good for 10 p.m. on May 1, 9 p.m. mid-month, and 8 p.m. on May 31. The map shows you where to find the Southern Cross; this is the ideal time to look for the Cross, when it is due south and thus at its highest point in the sky. It’s still quite low as seen in the islands; about six degrees above the horizon as measured from the bottom (and brightest) star, Acrux, to the horizon. To locate the cross, make sure you have a flat horizon such as the sea to view it.  The bright stars Alpha and Beta Centauri, to the left of the cross, are just rising at map-time and can be an invaluable guide. These two brilliant stars are called ‘The Southern Pointers.” Find them, look right, and they’ll point you to the top star in the Cross, Gacrux.

The Southern Cross will be visible, as every year, through the end of June; the latest I’ve ever seen it in the summer is July 4.  It then returns to the predawn sky by early December.

Moon phases (Hawaiian Time zone):

First Quarter: May 2
Full: May 10
Third Quarter: May 18
New: May 25
First Quarter: June 1

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2017-06-06T10:55:59+00:00Monthly Astronomy Article|