Summer officially begins Monday at 4:28 a.m.

The summer solstice represents the change from spring to summer, and the longest day of the year.

The Earth is like a giant gyroscope that spins completely around its axis once a day. As the Earth moves around the sun, its axis always points to the same place in the sky, but the relationship of the Earth to the sun constantly changes. The North Pole is tilted toward the sun for half of the year and away from the sun for the other half.

This changes the amount of sunlight a given place receives throughout the year. The Northern Hemisphere leans toward the sun during summer and leans away from it during winter. In the spring and fall, both hemispheres have about the same tilt with respect to the sun.

Mark your calendar

Alcon 2010, the Astronomical League's annual convention, takes place June 25-26 at the Tucson Hilton East, 7600 E. Broadway. The conference, which was last held in Tucson 25 years ago, is a must for anyone interested in astronomy.

It's hosted by the Tucson Amateur Astronomy Association, the International Dark-Sky Association and the Astronomical League, and features a number of world-class experts. Speakers include David Eicher, the editor of Astronomy magazine; Wally Pacholka, noted night-sky photographer; Richard Green, the director of the Large Binocular Telescope Observatory; and comet discoverer David Levy.

Roger Angel, the director of the University of Arizona's Mirror Lab, is a featured speaker. He won the 2010 Kavali Prize, which goes to eight scientists whose discoveries have dramatically expanded human understanding in the fields of astrophysics, nanoscience and neuroscience.

New this year will be solar telescopes and an exhibition hall packed with everything from telescopes and books to meteorites and jewelry. Geoff Notkin, one of the stars of the Science Channel's "Meteorite Men," is scheduled to attend.

Other events will include a lighting-ordinance workshop and the Lunt Solar Systems Arizona Solar Conference, and the annual general meeting of the International Dark-Sky Association. To register, go online to

Tim Hunter has been an amateur astronomer since grade school. E-mail him at