"The Eagle has landed!"
July 20 is the 53rd anniversary of the Apollo 11 landing, which took human explorers to the surface of the Moon for the first time.
Carrying Neil Armstrong and Edwin "Buzz" Aldrin, the lunar module "Eagle" touched down in 1969 on the western edge (yellow circle) of the Moon's Sea of Tranquility. This is a dark, irregularly shaped lava floodplain approximately 876 kilometers (544 miles) in diameter. Located on the Moon's eastern hemisphere, it's visible to the unaided eye between the first quarter and waning gibbous phases and is situated between the comparably-sized Sea of Serenity and the Sea of Fertility.
Summer showers 1: The Delta Aquarids
The annual Delta Aquarid meteor shower peaks during the early morning hours of July 28, coinciding with a new Moon. This means that if observers can get away from city lights, they may see about 20 meteors per hour between midnight and dawn, radiating from the stars of Aquarius the Water-Carrier.
During that viewing window, Aquarius is located low in the south-southeast at that time, between the prominent planets Jupiter and Saturn. Caused by dust scattered along the trail of Comet 96P/Machholz, this shower is active from roughly July 12-August 23, with a long, leisurely peak that may stretch across several days, eventually blending with the early stages of the Perseid shower of August.
- Duration: July 12-August 23, peaking on July 28
- Parent comet: 96P/Machholz
- Speed: 40 kilometers (21 miles) per second
- Peak rate 20 per hour on July 28 under ideal conditions
Giant planet jamboree I: Mars meets Uranus (oh, stop it…)
Planet-watchers with binoculars have a good opportunity to spot the seventh world from the Sun—Uranus—on August 1-2, when it's located only 1.5° north of the red planet Mars. (For comparison, that angle is about three times the width of a full Moon, or a little less than the width of your thumb, held at arm's length, and easily within the same field in a standard pair of binoculars.)
At 2.9 billion kilometers (1.8 billion miles) away, Uranus is a tiny, steadily shining, greenish disk just on the edge of naked-eye visibility on moonless nights, but binoculars will help make it easier to spot. It was discovered telescopically in 1781 by William Herschel, who thought that it was a comet. He changed his mind when he didn't observe the coma or tail characteristic of comets, and concluded that he had found a planet. Expressing his desire to name it after his patron, King George III, he proposed calling it "Georgium Sidus" (George's Star). This had little support among astronomers outside of England, who argued that the naming should follow the established convention of drawing from classical myth. However, both in keeping with that tradition and breaking from it, the new planet was named after a Greek deity rather than being given a Roman name like the other planets. That name—Ouranos, after the personification of the sky—evolved into the name used today...which many people still can't decide how to pronounce.
Summer showers 2: The Perseids
The Perseid meteor shower is usually one of the best celestial displays of the year, normally producing up to 100 meteors per hour under ideal conditions (moonless sky, no bright lights, rural location, radiant hypothetically straight overhead).
However, this year's peak coincides with a bright full Moon whose glare washes fainter meteors from view and, according to the authoritative American Meteor Society, significantly reduces the rate to about 10 per hour. Nevertheless, according to NASA's Meteoroid Environment Office, the Perseids more than any other shower can produce fireballs as bright as the planet Jupiter and may still be worth watching, even under a bright Moon.
- Duration: July 17-August 24 and slowly building to a peak on the morning of August 13
- Caused by dust particles from 109P/Swift-Tuttle.
- Recorded by Chinese observers in 36 AD, but not recognized as a recurring annual event until 1836.
- Among the faster meteors known, Perseid particles enter Earth's atmosphere at 60 kilometers (32 miles) per second—about the distance from San Francisco to NASA's Ames Research Center in Mountain View, covered in a heartbeat.
- The shower is also known in Christian tradition as the "Tears of St. Lawrence," considered a sign of the heavens' mourning over Lawrence's grisly execution (being burned alive, during which he is alleged to have quipped "I'm done on this side, turn me over," thus making him a patron saint of comedians). No, really.
Giant planet jamboree II: Nor are we neglecting Neptune
On September 16, the most distant and arguably the most ignored of the major planets—Neptune—is at opposition (opposite the Sun), rising at sunset on the boundary between the stars of the Zodiacal constellations Aquarius the Water-Carrier and Pisces the Fishes, 11° southwest of much brighter Jupiter.
Being the most distant world in the solar system, Neptune is not visible to the unaided eye, and in amateur telescopes is seen only as a tiny, bluish dot—not an easy object to find without a lot of experience or perhaps some computer-assisted guidance. In fact, it was the first planet to be found mathematically, when astronomers observing irregularities in the movement of Uranus concluded that the gravity of another as-yet-unknown body was the cause. Two astronomers—John Couch Adams and Urbain Le Verrier—independently calculated the mass and position of the object, which was observed telescopically by Johann Galle in 1846.
On September 22, the autumnal or fall equinox for Earth's northern hemisphere occurs at exactly 6:04 pm Pacific. On this day, the Sun rises due east and sets due west—halfway between the extremes of its yearly travel along the horizon as represented by its rising and setting points on the solstices. As seen from Earth, the Sun crosses the celestial equator in the sky (the imaginary extension of our planet's equatorial plane into space) and enters the southern half of the celestial sphere.
Although the term equinox ("equal night") implies that day and night are equal in length, that's only in theory and not quite the case in actuality because refractive effects caused by our atmosphere make the Sun visible above the horizon for about 8 minutes longer than at night. In Earth's southern hemisphere, where seasons are reversed, this day is the vernal or spring equinox, and the period of daylight starts lengthening.
Earth's axis of rotation, which is always tilted 23.5° from the vertical, is perpendicular to an imaginary line joining Earth and the Sun and is tipped neither toward nor away from the Sun.
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