July 4: A Subtle Shadow on the Moon
The full Moon passes through the pale, outer fringe of Earth's shadow, causing a barely perceptible penumbral lunar eclipse that is centered over South America. Penumbral eclipses are very subtle and are better-appreciated in photographic time exposures. For observers in San Francisco, the eclipse is in progress when the Moon rises at 8:33 pm PDT, with maximum at 9:29 pm, when the Moon is still very low—only eight degrees above the southeastern horizon and perhaps still obscured by murk along the horizon, which may obscure the slight, soft shading across the upper-left third of the Moon's disk.
Fun fact: Even if observers don't notice the partial eclipse, they can still enjoy a pretty consolation prize by about 10 pm, when the Moon is joined by the planets Jupiter and Saturn, forming a short, 15-degree long line in the southeast.
July 20: Footprints on the Moon
This day marks the 51st Anniversary of the Apollo 11 landing that took humans to the surface of the Moon for the first time. The landing site at the Sea of Tranquility and those of subsequent missions have been photographed by lunar-orbiting satellites, including the paths kicked up in the lunar dust by the astronauts' activities. Special reflectors left during several missions were targeted with lasers, enabling scientists to measure the precise distance to the Moon to within inches.
July 22: See All Five Naked-Eye Planets
About an hour before dawn, with Mercury at maximum western elongation and very low in the east-northeast, bright Venus 23 degrees to its upper-right (with the bright, orange star Aldebaran six degrees farther along). Ruddy Mars is unmistakable high in the south, and the giants Saturn and Jupiter are closely paired very low in the southwest. If your horizon isn't low enough to span the 168-degree angle between Mercury and Jupiter, start observing earlier, before Jupiter fades in the haze, then enjoy the sight of the other planets until Mercury rises.
Fun fact: On the 17th and 18th, the waning crescent Moon provides a bonus, located between Venus and Mercury. On those dates, the span of planets is a little less...but so is the separation between Mercury and the Sun, making the elusive, little planet even harder to find. But wait, there's more! There's actually a sixth planet in the mix, too—Uranus—located about halfway between Venus and Mars, but it's not visible without a telescope.
August 12: A Sprinkling of Stardust
One of the year's stronger meteor displays—the Perseid meteors—peaks during the predawn hours, although they're active for several weeks, slowly building from July 17 through August 24 to a peak rate of about 80 per hour under ideal conditions on August 12, then diminishing rapidly. This is caused by Earth's passage through a trail of dust left behind by Comet Swift-Tuttle, causing the dust particles to heat up and glow as they plummet through the atmosphere.
Coinciding with a Moon that is just past third quarter, this display will be obscured by moonlight, reducing the number of meteors seen. To optimize your chances of seeing meteors, get away from bright lights as much as possible, allow your eyes at least 20 minutes to adjust to the dark, find a comfortable observing spot where you have a wide-angle view of the sky, and plan on watching for a couple of hours. Although the point from which the meteor-streaks appear to radiate, or radiant, is in the constellation Perseus, rising in the northeast around midnight, don't look just in that one part of the sky, or you'll see mostly the particles coming toward you, which will have very foreshortened trails. If you look farther from the radiant, you'll see longer streaks.
Fun fact: The shower's parent comet is named after its co-discoverers, astronomers Lewis Swift and Horace Tuttle. A television news anchor who did not understand the nomenclature mistakenly referred to the comet as "the Swift Turtle."
Sept 22: "I Don't Fall...I Do Random Gravity Checks"
Autumn arrives at 6:31 am PDT, marking the start of Autumn (or Fall) in the Northern Hemisphere. In the Southern Hemisphere, where the seasons are reversed, it's the start of Spring. To avoid confusion, many simply refer to it as the September equinox and ignoring the season, making it applicable on both sides of the equator.
- The Sun is located on the celestial equator which intersects the horizon exactly due east and due west. This causes the Sun to rise due east and set due west.
- Although "equinox" means "equal night" and implies that day and night are of equal length on this day...they're not. Refractive optical effects caused by Earth's atmosphere create mirages that make the Sun appear when it's still below an observer's horizon and linger in the sky after it has actually set. This—along with whether we define sunrise and sunset to mean the top or center of the Sun's disk is on the horizon—means day seems longer than night by about eight minutes.
- It's not true that the equinox is the only time you can balance an egg on its end because "the gravity of the Sun and Moon are balanced," or something like that. Gravity doesn't work that way, and neither do eggs. You can balance eggs on their ends on any day of the year, if you're careful enough.
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