Half Moon over Casa Grande, Arizona

The saguaro cactus in this photo was in full bloom and about 30 feet tall. These cacti bloom in late April and May and they only grow in the Sonoran Desert in California, Arizona, Baja California and half of the state of Sonora, Mexico. Saguaros must live below an elevation of about 3,500 feet due to the danger of frost. In this photograph, the half moon is technically in its first quarter phase. This phase occurs when the positions of the sun, earth and moon in their orbits form a right angle. From earth we always see the same face of the moon, as the moon does not rotate. Over millions of years, the effect of the Earth's gravity has slowed down the Moon's rotation until it became gravitationally locked to the Earth.