Highlights of the Season: July-September 2019

On July 2, the Moon's shadow falls across Earth's surface, causing a total solar eclipse. Most of the Moon's shadow path falls across the southern Pacific Ocean, with landfall only on the tiny, uninhabited island of Oeno, and then in Chile, 5770 kilometers (3500 miles) away. The shadow continues across central Argentina to the Atlantic coast before leaving Earth's surface. While observers aboard chartered eclipse-chasing flights or ships in the Pacific would be able to see it early, observers on the South American mainland don't see it until late in the day. The shadow's narrow path is only 145 kilometers (90 miles) wide and passes over the cities of La Serena, San Juan, and Rio Cuarto, and the southern outskirts of Buenos Aires. Eclipse-watchers on that path using safe viewing methods will see the silhouette of the new Moon move slowly across the Sun's face until totality, when it completely hides the solar disk from view for about 2½ minutes just before sunset. Only in Chile will the entire eclipse be seen from beginning to end. In Argentina, although all of totality will be seen, the Moon will still be in the late partial phases by sunset. Observers in most of the rest of South America will see at least a partial solar eclipse, but not totality if they're not on the narrow shadow-path. None of the eclipse will be visible from any part of the United States.

On the morning of July 4, 1054, Chinese and Native American skywatchers chronicled the appearance of a "guest star" among the stars of what we now call Taurus the Bull. This object gradually brightened, becoming visible in the daytime sky for about three weeks before fading from view. It remained perceptible in the night sky for two years before finally disappearing from view (and remember, this is centuries before the telescope was invented). What observers saw was the catastrophic explosion of a star, also called a supernova, and modern astronomers have photographed what remains—a tattered cloud of gas known as a supernova remnant, blasted off the star. Dubbed the Crab Nebula, it currently measures about 11 light years across, but is still expanding at a rate of about 1500 kilometers (1000 miles) per second. At its heart is the remaining collapsed core of the massive pre-supernova star.

On July 16, the full Moon skims the edge of Earth's inner shadow, or umbra, causing a partial lunar eclipse that is seen from most of Africa and central Eurasia. Centered over the Mozambique Channel, the eclipse is visible in its entirety from Eastern Europe, much of Africa, the Middle East, eastern Eurasia, and Antarctica. Some part of it is visible from South America, Australia, and eastern Asia. Since the Moon never completely enters the umbra, at maximum eclipse, Earth's shadow falls two-thirds of the way across the Moon's diameter, leaving a broad crescent on its southern side still brightly illuminated by sunlight.

On July 20, 1969, humans walked on the surface of a body off the planet Earth when the Apollo 11 lunar module "Eagle" touched down on the Moon's Sea of Tranquility, with astronauts Neil Armstrong and Edwin "Buzz" Aldrin on board. After a day on the lunar surface during which they took photographs, performed experiments, and collected samples of rock and regolith, the astronauts blasted off and returned to Earth, splashing down safely in the Pacific Ocean on July 24. This accomplishment was repeated five more times by 1972, and the landing sites of each mission have been imaged by NASA's Lunar Reconnaissance Orbiter.

The Perseid meteor shower, the most popular meteor shower of the year for skywatchers in the northern hemisphere, fortuitously takes place during usually warm summer months, and many people plan camping trips around it. The shower peaks on the morning of August 12, although the light of a waxing gibbous Moon (three days before full) interferes with this year's display. As Earth passes through dust left in the trail of Comet Swift-Tuttle, the tiny particles (about the size of grains of sand) plummet through the atmosphere at around 18 miles per second. At that speed, air molecules can't get out of the way, piling up in front of the particle, and the compression heats the air up to about 3000 degrees Fahrenheit, incinerating the dust particle in a flash of light. Under ideal conditions—i.e., on a moonless night and observed away from city lights—observers might see about 60-80 meteors per hour radiating from the direction of the constellation Perseus the Hero. A good strategy, especially given this year's Moon phase, is to observe for a few nights, not just on the night of the peak. Historically, the Perseids were also known to Catholics as the "Tears of St. Lawrence," since their peak occurs near August 10, which is the anniversary of that patron saint's martyrdom.

Five planets are visible in the sky with the unaided eye. If you include Earth's Moon, that makes six objects. On August 29 and 30, only two of them can be seen—Jupiter and Saturn. Where are the other planets (Mercury, Venus, and Mars)? They're all clustered near the Sun and buried in its bright glow. If you include the Moon, which is new at that time, that means four of the Solar System's six naked-eye objects are not visible. After the waxing crescent Moon reappears in the sky after sunset on August 31 and continues its monthly trip around the sky, the planets will remain hidden in the Sun's glare through September.

Why is September's full Moon on the 13th called the Harvest Moon? It has to do with the time of moonrise from one evening to the next. Normally, the Moon rises about 50 minutes later from night to night, but as seen by observers in the Northern Hemisphere, the full Moon occurring nearest the autumnal equinox on September 23 rises only about 30 minutes later each successive night, meaning there are a few days in a row when the bright Moon rises close enough to sunset to provide continuous, usable light immediately after sunset for farmers to continue harvesting crops in the field.

The September equinox (AKA the fall or autumnal equinox) occurs on September 23 at 12:50 am PDT. When the Sun rises that morning at 6:58 am PDT, it does so exactly due east, and sets due west at 7:05 pm PDT, 12 hours 7 minutes later. But isn't the equinox when daytime and nighttime are both exactly 12 hours long? The conventional wisdom says so, but that doesn't account for refraction by Earth's atmosphere, which causes a mirage of the Sun to be seen before it actually rises and, likewise, to be seen after the Sun actually sets. This extends the time that the Sun is visible above the horizon by several minutes. The theory also assumes that the Sun is represented by a point at its center, rather than by a half-degree wide disk. Is "sunrise" or "sunset" when the top of the Sun's disk is on the horizon or the center of the disk? Depending on the context of its usage, the way the word is defined may confuse things a bit further and make an additional minute's difference.