In the tropics, there are two days a year when the sun hangs exactly overhead. The term "Lahaina Noon" was chosen in a contest that Bishop Museum sponsored in 1990 to provide a convenient and local name for this phenomenon.

In Honolulu, the first Lahaina Noon came on May 27; the second will be on July 16. At 12:37 p.m. on that day, the sun will be exactly overhead and any upright object such as a flagpole will have no shadow.

This phenomenon occurs only in the tropics. Throughout the main Hawaiian islands the specific dates vary by latitude because the farther north in the islands, for example, the earlier July’s Lahaina Noon day will be. It occurs in Lihue at 12:42 p.m. July 11 and in Hilo at 12:26 p.m. July 24.

Bishop Museum’s planetarium website provides the days and times for many other locations in the islands as well: www.bishopmuseum.org/planetarium/planetarium.html.

The museum will offer a special planetarium program about Lahaina Noon at noon July 16. It is included in the museum’s admission price and lasts about 30 minutes. The planetarium show will be followed by viewing the actual Lahaina Noon outside.

PLANETS IN JULY

For all of July we have a special guest in the night sky. Like a promising understudy, the planet Mercury will appear in the role usually performed by Venus. Look for Mercury in the west around 8 p.m., which is when it gets dark this time of year in Hawaii. The planet will be about 10 degrees above the western horizon, which is the width of your fist held at arm’s length. You’ll only have 30 minutes to see Mercury, since it sets around 8:30 p.m. through July. Mercury starts the month shining brightly at minus 0.4 magnitude, and then fades to first magnitude by the end of July.

It probably will be easiest to spot Mercury on July 2. On that night, a slender crescent moon will be just to the left of Mercury. So find the moon first, and then look to the right of the moon. That really bright dot will be Mercury. The moon and Mercury both set at around 8:30 p.m. July 2.

Mercury is the closest planet to the sun, much closer than the Earth. This means that Mercury never seems to be very far away from the sun. For this reason, you only see Mercury rising just before the sun in the morning, or setting just after the sun in the evening.

Saturn shines in the western evening sky throughout July. The planet shines at first magnitude, slightly brighter than the star Spica, which is about 15 degrees (1.5 fists) to the left of Saturn. Spica has a slight bluish color, Saturn a slightly yellowish color.

At the start of July, look for Saturn about two-thirds of the way up in the west at dusk; the planet will set around 12:30 a.m. In the middle of the month Saturn is halfway up the western sky at dusk and sets at 11:30 p.m.; and by the end of the month, it’s about one-third of the way up in the sky at dusk and sets around 10:30 p.m. Look for the first quarter moon next to Saturn on the nights of July 7 and 8.

Jupiter, the king of the planets, rises in the east at 2 a.m. at the start of July and at midnight by the end of the month. At minus 2.4 magnitude, you can’t miss it in the early-morning sky; just look east for the brightest dot of all, and that’s Jupiter. Jupiter’s only rival in brightness, Venus, is out of commission until September. Look for the waning moon next to Jupiter on the mornings of July 23 and 24.

Mars is still a fairly dim object in the morning sky. It rises in the east around 3:30 a.m. at the start of July and around 3 a.m. at the end. Look for the waning crescent moon close to Mars on the morning of July 27.

SKY MAP FOR JULY

Scorpius dominates the southern sky. The distinctive constellation does look like a Scorpion. The Summer Triangle is high in the east. These are three bright stars from three different constellations that form a giant star triangle: the star Altair in Aquila the Eagle, Vega in Lyra the Harp, and Deneb in Cygnus the Swan.

Look for the Big Dipper low in the northwest. The Big Dipper is an asterism rather than an official constellation. An asterism is a distinctive star pattern that is not, in itself, one of the 88 official constellations. The Big Dipper is made up of seven bright stars, and makes up part of a larger constellation called Ursa Major, the Big Bear.

The pointer stars of the Big Dipper — the two stars in the cup that don’t have the handle attached — point to the North Star. The North Star stays almost exactly in place throughout the night.

Cassiopeia is on the other side of the North Star from the Big Dipper. It’s rare to see both the Dipper and Cassiopeia at the same time in Hawaii, but it is possible here in the July evening sky. Cassiopeia is just rising in the east and has a distinctive "W" shape.

The Big Dipper is a good guide to other stars as well. The Dipper’s handle is bent, or has an arc to it. If you follow the handle, it points you to the bright star Arcturus ("arc to Arcturus" as the saying goes.) Arcturus is halfway up in the western sky on this map. You may know it better by its Hawaiian name, Hokule‘a ("star of gladness"); the famous voyaging canoe is named for the star.

This is the season for Hercules. Hercules the Kneeler is high overhead. The stars of Hercules are not bright, and it takes some hunting to find the constellation. The core of the constellation is a butterfly shape of six stars that look like a watered-down version of Orion.

In the north, the constellation of Draco winds its way between the Big and Little Dippers.

Mike Shanahan is director of education, exhibits and planetarium at Bishop Museum. Reach him at mikes@bishopmuseum.org.