A trio of planets, all visited by NASA robotic missions, is visible in the summer sky this week. Mars and Saturn can be spotted low in the west-northwest for a couple hours after sunset. Jupiter rises low in the southeast at sunset.
On July 10, Saturn and Mars appear so close together that they can be spotted in the same binocular view. They are not actually close to each other except in our line of sight. Saturn is about 1.5 billion kilometers (about 930 million miles) from Earth, and Mars is about 330 million kilometers (about 205 million miles) away. Saturn is largest of the two and, therefore, brighter. Note the subtle color difference between creme-colored Saturn and ruddy Mars.
Jupiter reaches opposition on July 9. Opposition is when the sun and another heavenly body are on opposite sides of Earth. We see an opposition every month, when the sun and the moon are on opposite sides of Earth. When the sun sets, the full moon rises, and when the sun rises, the full moon sets. As the sun sets in the west, Jupiter rises in the southeast. At opposition, when Jupiter is closest to Earth, it is a mere 774 million kilometers (481 million miles) away.
Though visible with the unaided eye as a bright "star-like" object, Jupiter and its wonders are best revealed through a telescope. The largest planet in our solar system, Jupiter is a giant ball of hydrogen and helium cloaked in multicolored clouds. Through a telescope the alternating stripes of dark belts and light zones are visible.
Often up to four of Jupiter's moons can be seen lined up on the east and west of the planet. Three of the four -- Io, Ganymede and Callisto -- are larger than our own moon. Only Europa is slightly smaller. These moons' orbits around Jupiter take from three days for Io up to 14 days for Callisto. So on any given night, some of the moons are lined up east or west of Jupiter's equator while others are hidden in front of or behind the planet. Look again the next night and there will be a different lineup.