If you wax your car or exercise once in a blue moon, prepare to buff and lace up your sneakers: There will be a blue moon July 31.

July features two full moons. The first happened yesterday and the next full moon will be July 31 (2:05 p.m.). The blue moon nickname goes to the second full moon to occur within the same calendar month, according to David R. Williams of the NASA Goddard Space Flight Center in Greenbelt.

Blue moons happen every 21/2 years, according to Williams. The last one was in November 2001, and the next will be in June 2007. There were two blue moons in 1999, in January and March. February 1999 had no full moons.

There is debate among astronomers and an emerging controversy on defining blue moons, not solely as two moons in a given month but perhaps counting the full moons in a given season.

Beyond the moon, Venus begins to dazzle morning sky gazers. After crossing the face of the sun June 8, the gorgeous, bright planet ascends the pre-dawn heavens, climbing higher with each passing July day.

Look to the east-northeast just after 5 a.m. to find Venus low on the horizon. As the hour moves toward sunrise, Venus is easily spotted. It looks like a distant airplane with its landing lights on. This brilliant planet -- at negative 4.5 magnitude (very bright) in the middle of July -- hangs out near the star Aldebaran in the constellation Taurus. The sun, of course, erases it from the sky after dawn.

By mid-July, Venus rises just after 4 a.m., still in the east-northeast, giving viewers ample time to enjoy the spectacle. At month's end, the planet climbs the horizon beginning about 3:50 a.m., essentially giving Washingtonians a pre-dawn show that lasts hours. This planet will be the top morning goddess for several months.

To spy the other planets, you will have to wait until dusk. Jupiter, Mars and Mercury frolic after sunset in the western sky. Among those three planets, Jupiter will be the easiest to spot as it is relatively higher and bright. It is in the constellation Leo (west-southwestern sky), and this big, gaseous planet is negative 1.8 magnitude, bright enough to see through the hazy summertime heavens.

Mars and Mercury are closer to the horizon this month. Mars, an extraordinarily bright object last summer, has faded to a positive 1.8 magnitude (very dim). To make matters worse, it is hard to see as it skims the western horizon before setting.

Mercury becomes a little higher than Mars in late July. In fact, Mars and Mercury conjoin July 10.

Down-to-Earth Events

* July 5 -- Astronomer Amy Simon-Miller discusses the Cassini Mission's Saturn orbit insertion at an open house at the University of Maryland observatory in College Park. Telescope viewing after the talk, weather permitting. 9 p.m. Information, 301-405-6555; www.astro.umd.edu/openhouse.

* July 7 -- Geophysicist Bruce Campbell on "NASA's Return to the Moon," at the National Air and Space Museum. Meet at the museum seal. Noon. Information, 202-357-2700; www.nasm.si.edu.

* July 14 -- Historian Allan Needell on preserving the Saturn V moon rocket, at the National Air and Space Museum. Meet at the museum seal. Noon. Information, 202-357-2700; www.nasm.si.edu.

* July 16 -- Celebrate Mars Day at both the National Air and Space Museum and the museum's Steven F. Udvar-Hazy Center near Dulles International Airport, 10 a.m. to 3 p.m. Information, 202-357-2700; www.nasm.si.edu.

* July 17 -- Enjoy the real heavens at Sky Meadows State Park, near Paris, Va. Dusk to 11 p.m. $4 parking fee. Information, 540-592-3556; www.dcr.state.va.us/parks/skymeado.htm.

* July 20 -- Astronomer Sylvain Veilleux talks about "Supermassive Black Holes" at an open house at the University of Maryland observatory in College Park. Telescopic sky-viewing after the talk, weather permitting. 9 p.m. Information, 301-405-6555; www.astro.umd.edu/openhouse.

* July 24 -- Hunt the heavens at "Exploring the Sky," presented by the National Capital Astronomers and the National Park Service, at Rock Creek Park near the Nature Center, in the field south of Military and Glover roads NW. 9 p.m. 202-895-6070; capitalastronomers.org.

* July 28 -- Historian Cathleen Lewis answers the question, "How Do They Go to the Bathroom in Space?" at "The Russian Space Toilets," National Air and Space Museum. Meet at the museum seal. Noon. Information, 202-357-2700; www.nasm.si.edu.

Blaine Friedlander may be reached at bfriedlander@earthlink.net.