Summer kicks off, and three planets gather to celebrate
June 16, 2005
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June 16, 2005
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Break out the sunscreen — summer officially starts in the Northern Hemisphere June 21. Just 3 days later, Mercury, Venus, and Saturn all huddle together in the early evening sky.
The summer solstice
The summer solstice is the longest day of the year — the most hours of daylight — for people living north of the equator. This astronomical event occurs officially at 2:46 A.M. EDT, when the Sun reaches its highest point above the celestial equator, a projection of Earth's equator into space. Since the winter solstice 6 months ago, when the Sun was at its lowest point in the sky, our star has been climbing steadily each day, increasing the amount of daylight.
For several days preceding and following the solstice, however, the Sun appears to stand still, its maximum altitude unchanging. This phenomenon is captured by the word "solstice." In Latin, sol means "Sun," and stitium is a suffix that means "stop."
Although the solstice is the longest day of the year, it isn't necessarily the hottest. A lag in solar heating means temperatures will continue to rise for about a month afterward. So, while the dog days of summer will be here soon, June 21 actually marks the start of the Sun's descent in the sky, leading to autumn and winter's increasingly shorter days and longer nights.
These seasonal changes occur because Earth's axis, the imaginary line connecting the North and South poles, tilts 23.5° with respect to the planet's orbital plane. In summer, when the Northern Hemisphere is tipped toward the Sun, it receives radiation more directly than it does during winter, when the Sun is lower in the sky.
So, while our "summer" solstice brings many people north of the equator happiness, people in the Southern Hemisphere are in the midst of winter.
Three planets visible in the early evening sky
In the west-northwest evening sky, June 21, Mercury and Venus will be just 2° apart (equal to 4 Moon-widths) with nearby Saturn to their upper left. By June 24, Mercury lies within 1° of Venus. The two innermost planets will continue to approach one another visually until June 27, when they'll be separated by just a few arcminutes (less than half a Moon width) — Saturn then will lie to the lower right of the Mercury-Venus pair.
Binoculars are recommended to see these three planetary delights, as they'll be close to the horizon 30 minutes after sunset. The bright stars Pollux and Castor, both in the constellation Gemini the Twins, appear to the right of the three planets.
The three planets appear closest together in late June, but you may need binoculars to pick them out of the twilight.
Photo by Astronomy: Roen Kelly
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