Skywatch July 2017

Skywatch July 2017

While we’ve lost the Southern Cross from the Hawaiian Islands by the time July starts (not to return till December), July does provide a last chance to catch the brilliant stars Alpha and Beta Centauri, the best summertime examples of stars we see in Hawaii and don’t see from most of the rest of United States. Along similar lines, the overhead sun returns in Hawaii in July, a phenomenon unique to the tropics and which never occurs in the other 50 states.

Return of the overhead sun

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. Since Hawai‘i is the only tropical state, it’s also the only state that experiences the overhead sun.

Throughout the main Hawaiian Islands, the first overhead sun of the year occurs in May and the second in July. The date varies depending on how far north you are from the equator. Thus, for Līhu‘e on Kaua‘i, the July overhead sun occurs on July 12. Further south, on the Island of Hawai‘i, the overhead sun occurs nearly two weeks later, on July 24, in both Hilo and Kailua Kona.

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 from the top of the sky.

The chart below gives the July overhead sun dates for a number of locations.

2017 Lāhaina Noon days and times

LīhueJuly 12 12:42 p.m.
Kāne‘oheJuly 15 12:37 p.m.
Honolulu July 16 12:37 p.m.
KaunakakaiJuly 16 12:34 p.m.
Lāna‘i CityJuly 18 12:34 p.m.
LahainaJuly 18 12:33 p.m.
KahuluiJuly 18 12:32 p.m.
Hana July 18 12:30 p.m.
Hilo July 24 12:27 p.m.
Kailua-KonaJuly 24 12:30 p.m.
South Point Island of Hawai‘iJuly 27 12:28 p.m.

 

July Star Map: dueling gas giants

Since many of the great sights of our July sky are seen clearly on our July 2017 sky map, I’ll use that map as our point of reference as we discuss how summer celestial sights.

As with all of our monthly star maps, the July map is good for 10 p.m. early in the month, 9 p.m. in the middle of the month, and 8 p.m. at the end of the month.

Jupiter

At map-time, look for the bright white dot of Jupiter about one-third of the way up in the western sky. Jupiter is the brightest dot in evening sky. Jupiter shines at minus 2 magnitude, much brighter than any star, so it pops out easily. The blueish-white star Spica, about ten degrees (the width of your palm at arm’s length) to the left of Jupiter, shines respectably at 0.96 magnitude, but Jupiter outshines it by 20 times.

In early July, Jupiter will set in the west at 12:30 a.m.; by the end of the month, by 10:45 p.m. On Saturday July 1, look for the just-past-first-quarter moon making a triangle with Jupiter and the star Spica.

Saturday July 1 is also the evening that we’ll doing on evening program Stars and Guitars for July. The planetarium program runs from 8:00 to 9:00 p.m.; once it’s over, weather permitting, we hope to catch both the moon, Jupiter, and some of Jupiter’s moons in our observatory telescope.  Information on, and reservations for, Stars and Guitars: https://www.bishopmuseum.org/stars-guitars/

Saturn

If you return to our star map, Saturn is about halfway up in the southeast at map time (which again is 10 p.m. in early July, 8 p.m. in late July). Saturn has a distinctive yellow-white color. While it’s nowhere near as bright as Jupiter, Saturn does shine nicely at 0 magnitude. This makes Saturn about 2.5 times brighter than the star Antares in the Scorpion, the first-magnitude star to the right of Saturn.

In early July, Saturn will be due south at 11:30 p.m. and will set in the west at 4:45 a.m., just as the first light of dawn starts to break. By the end of the month, Saturn is due south at 9:20 p.m. and sets by 2:30 a.m.

Saturn’s rings are perhaps the most beautiful telescope sight of all. The rings should be even more striking than usual this season. The rings are tilted 27 degrees to our line of sight this summer, the best ‘tilt’ since 2003; this means that the rings are exceptionally well-angled for good telescope viewing.

In addition to Jupiter and the moon, we hope to catch Saturn and its rings in our observatory telescope after the ‘Stars and Guitars’ program on Saturday July 1. Our Stars and Guitars planetarium program happens rain or (moon) shine, though our telescope is weather-dependent! Reservations: https://www.bishopmuseum.org/stars-guitars/

Star map constellations

Keah Watkins | Bishop Museum

In addition to featuring good views of Jupiter and Saturn, the July star map shows you how to find many well-known constellations.

The Southern Pointers – stars you can’t see from most of the rest of the USA

While the Southern Cross has left our sky, as it does every July, the Southern Pointers (Alpha and Beta Centauri) can still be seen on the map, very low in the South. The brighter of the two, Alpha Centauri, is the third-brightest star of all, and the brightest star you can see in the July evening sky. Beta Centauri is the 11th brightest star. Alpha Centauri shines at minus 0.27 magnitude, Beta Centauri at 0.6.

With the Southern Cross gone, Alpha and Beta Centauri are the best examples in our summer sky of stars that can be seen from the Hawaiian Islands but cannot be seen much of the rest of the northern hemisphere. These two stars are not visible from any location that is north of 29 degrees latitude, which is the latitude of Houston, Texas.  Whether you’re in Los Angeles or Atlanta or Boston or Chicago (or Tokyo or Beijing), you’ll never see Alpha and Beta Centauri in your home skies.

The constellation of the Scorpion, or Maui’s Fishhook, is due south on our map. It is one of the most striking shapes in the sky; its brightest star, Antares, has a distinct orange tint to it. To the left of Scorpius is Sagittarius, the Archer, aka “The Teapot.”

Our map shows the star Arcturus high in the western sky. The star is known as Hokule‘a in Hawaiian, ‘the star of gladness. You can also see the Big Dipper in the north, its cup pointing to the North Star and its handle pointing to Arcturus.

The summer triangle is rising in the northeast by map time; the triangle is made of the bright stars Vega, Altair and Deneb, pulled from three separate constellations to make a big triangle in the sky.

 

Other sky events in July:

 

Venus lights up the hours before dawn

Throughout July, Venus rises in the east right around 3 a.m. as is about 1/3 of the way up in the east at daybreak. Venus shines at minus 4 magnitude, many times brighter than even Jupiter. Venus will remain in this role as a predawn beacon all the way till the end of October 2017.

 

Mercury:

The elusive planet Mercury is visible for the entire month of July 2017, though it clings to the west horizon at dusk all month. In early July the planet is a mere four degrees above the west horizon (the width of two fingers at arm’s length) about 7:50 p.m., when it gets dark enough to see it.  By 8:10 p.m. From mid-month to the end, the planet is nearly ten degrees (a whole palm) above the west horizon when it gets dark around 7:50 p.m., giving you almost an hour to find it before Mercury sets at 8:40 p.m. Mercury starts the month at brilliant minus 1 magnitude and dims to 0.47 magnitude by July 31.

Mars: Mars is not visible at all in July.

Southern Delta Aquariid Meteor Shower

While not one of the more striking showers, this meteor shower in late July does occur this year during a time when the moon is out of the way, which will improve the viewing odds.  The peak time: the early hours of the morning on July 30, i.e. from 12:01 a.m. to dawn on that day, though all mornings from July 27 through August 1 should show increased meteor activity. Search the eastern and southern skies for streams of light; under ideal sky conditions, viewers can see up to 25 meteors an hour.

Orion’s return

Orion returns to the eastern predawn sky by July 15, after its usual mid-May to mid-July absence from our sky.

Moon phases (Hawaiian Time zone):

First Quarter: July 1, July 30
Full: July 9
Third Quarter: July 16
New: July 23

2017-07-05T09:40:44+00:00 Monthly Astronomy Article|