May should be a great sky-watching month in Hawaii. Mars will be brighter and closer this May than any time in the past 10 years; Jupiter and Saturn dance in the midevening sky; Mercury transits the sun on May 9; and throughout the islands, the first of the year’s two “overhead sun” days occur, a phenomenon unique to the tropics.

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 around the far side of the sun, 230 million miles from Earth. At its closest, Mars, a mere 47 million miles away, shines almost as brilliantly as Jupiter.

During the peak nights on May 21-30, Mars will shine brighter than minus 2 magnitude, and will peak on May 22-23 at minus 2.08, brighter than any star.

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 a straight line between the sun, Earth and Mars, with Earth in the middle. The moment of Mars’ opposition occurs at 1:17 a.m. May 22. You can compare Mars’ 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, Jupiter is just slightly brighter at minus 2.11.

When a planet is in opposition, it rises in the east as the sun sets in the west; it is high in the south at midnight; and it 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 is also overhead all night long.

Interestingly, the lineup of the sun, Earth and Mars is such that the red planet’s closest approach to Earth is actually not during opposition, but instead on May 30. Mars will still be brilliant on that date, at minus 2.03, and the planet will be 46.8 million miles away.

In fact, Mars will be brilliant all month. 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.

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 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. 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 it strong competition this May. Throughout the month, Jupiter lies just below Leo the Lion. In early May, look for Jupiter two-thirds 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 high in the southwest at dusk, and sets around 1 a.m.

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 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 (10 degrees — the width of a fist at arm’s length).

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

In Hawaii, this transit begins at 1:15 a.m. May 9, hours before the sun rises. On that day, Mercury will already be two-thirds 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) until the transit ends at 8:40 a.m. You’ll need a telescope with a good solar filter to view it.

Lahaina noon

In the tropics, the sun passes directly overhead twice a year. During the noon hour on these days an upright object such as a flag pole will have no shadow.

The exact times and dates of the “overhead sun” vary depending on how far north or south you are in the tropics.

In Hawaii we call this a “Lahaina noon,” a term selected by Bishop Museum in a 1990 contest held to select a name for the zenith noon phenomenon. “La haina” means “cruel sun” in Hawaiian.

Here are the 2016 overhead sun dates and times for May and July, the two months each year when the main islands experience the phenomenon:

Lihue, May 30 at 12:35 p.m., and July 11 at 12:42 p.m; Kaneohe, May 27 at 12:28 p.m., and July 15 at 12:37 p.m.; Honolulu, May 26 at 12:28 p.m., and July 15 at 12:37 p.m.; Kaunakakai, May 25 at 12:25 p.m., and July 16 at 12:34 p.m.; Lanai City, May 23 at 12:24 p.m., and July 18 at 12:34 p.m.; Lahaina, May 24 at 12:23 p.m., and July 18 at 12:33 p.m.; Kahului, May 24 at 12:22 p.m., and July 18 at 12:32 p.m.; Hana, May 23 at 12:20 p.m. and July 18 at 12:30 p.m.; Hilo, May 18 at 12:16 p.m., and July 24 at 12:27 p.m.; Kailua-Kona, May 18 at 12:20 p.m., and July 24 at 12:30 p.m.; and South Point, May 15 at 12:19 p.m., and July 28 at 12:28 p.m.

Mike Shanahan is director of Education, Exhibits and Planetarium. For more information, go to bishopmuseum.org/planetarium.html.