Sunrise & Sunset Table
Sunrise | Solar Noon | Sunset
5:51 am | 1:13 pm | 8:36 pm
Sunrise | Solar Noon | Sunset
6:13 am | 1:16 pm | 8:18 pm
Sunrise | Solar Noon | Sunset
6:39 am | 1:09 pm | 7:39 pm
(All times are PDT for San Francisco, CA, and will vary slightly for other locations.)
July 4 is typically the date given for the first observation of the "Crab Supernova" in the year 1054. Chinese skywatchers recorded the appearance of a "guest" star that grew bright enough to be seen in the daytime for nearly three weeks, after which it slowly faded, remaining visible in the nighttime sky for nearly two years—and remember, this is long before telescopes were being used.
Thanks to careful record-keeping by the Chinese, modern astronomers know exactly where to look to observe the location of this transient phenomenon, and they see the "Crab Nebula." This is the tattered shell of a star that exploded, also called a supernova. At its center is a pulsar, the star's dense, surviving core, originally between 1.5 and 3 times as massive as the Sun, but now compressed to about 20 kilometers (12 miles) across and rotating at the amazing rate of about 30 times per second. Even more amazing is the fact that other pulsars have been found that rotate 600 times per second!
Another July 4 Anniversary
Also on July 4, 1997, the Mars Pathfinder landed on an ancient Martian floodplain, making the first successful landing on Mars since Viking 2 in 1976. It used a radical new technique involving protective airbags that allowed the spacecraft to "bounce down" after parachuting to a low-enough altitude, rather than perform a powered descent.
Riding to the surface was the "Sojourner" rover, about the size of a microwave oven. The first wheeled robot on Mars, Sojourner was named after abolitionist and women's rights activist Sojourner Truth by then 12-year old Valery Ambroise in a nationwide NASA-sponsored essay contest.
Designed for a mission lasting seven Martian days (or "sols"), the plucky rover exceeded expectations and was active for more than ten times that—83 sols. Cautiously moving at a top speed of about 0.4 meters (15 inches) per minute, Sojourner traversed a total of about 100 meters (330 feet) throughout the mission and made its final contact with Earth on September 27, 1997.
Notice Mercury's angular separation from the Sun at eastern elongation on July 11, and compare it to its angular separation at western elongation on August 26. They're not equal—one is 26 degrees and the other 18. One might expect that simple symmetry would make them appear equal. However, Mercury's orbit is the most elliptical (or "out-of-round") of any planet in our solar system, causing its distance from the Sun to vary from 46 to 70 million kilometers (29 to 43 million miles), never very far out of the Sun's glow.
On July 20, 1969, the Apollo 11 lunar module Eagle descended to the surface of the Moon and gently touched down on the Sea of Tranquility, where Neil Armstrong and Edwin "Buzz" Aldrin became the first humans to set foot on a celestial body off Earth. Their footprints are still there, preserved in the airless environment of the lunar surface.
Although the individual footprints can't be seen, the wandering trails of disturbed lunar soil along the paths the astronauts walked are clearly evident in high-resolution images taken by spacecraft orbiting the Moon. This year, we observe the 49th anniversary of that historic achievement, even as various space agencies announce their plans to return humans to the Moon, with India having announced a robotic mission to the lunar south pole before the end of this year.
An Otherworldly Extravaganza
All five naked-eye planets are visible during the same evening in mid-July, spread out in a long line that stretches about 160 degrees. Unfortunately, for most observers in the U.S. this planetary parade is low in the south, which cuts off a planet at one end or the other.
The farther south observers are located, the higher the line crosses the sky, making more of it visible. From the latitudes of Key West, FL (24.5N) or Honolulu, HI (21.3N), it crosses about 45 degrees high, with Mars and Mercury at the ends, just above the eastern and western horizons, respectively. On the other hand, as seen from the Tropic of Capricorn—or about 23 degrees south latitude—the line crosses straight overhead, providing a full 180 degrees for all five planets to fit, with about an extra 10 degrees of space on either side.
See our Planet Watch page to find out when the Moon will be seen near each planet along this line.
Mars reaches opposition on July 26, when it's opposite the Sun in our sky and rises at sunset, making it visible all night. This is when Earth and Mars are aligned on the same side of the Sun and closest to each other. However, this opposition won't be quite as close as it was during the Red Planet's exceptional 2003 approach, when it was only 55.7 million kilometers (34.6 million miles) away—its closest position in nearly 60,000 years. Still, this time around, the two planets will be separated by 57.3 million kilometers (35.8 million miles) and won't be this close together again until 2035.
Mars will shine like an orange ember against the stars of Capricornus the Sea-Goat, with the almost-full Moon very close by on the night of the 26th. Where should people go to see Mars? Go outside after sunset and look southeast. This is not the kind of phenomenon like a solar eclipse which can only be seen from certain places on Earth, or a meteor shower which requires ideal conditions on a moonless night away from city lights. Mars will be bright enough to be easily seen from within the city, and many amateur astronomers will undoubtedly be out with their telescopes, which will be needed to see the disk of the Red Planet.
Although they overlap, their peaks are about two weeks apart, on July 28 and August 12, respectively. However, this means the two showers peak during opposite halves of the Moon's phase cycle and take place under conditions that are reversed from one another—the Delta Aquarids only a day after a bright full Moon and the Perseids a day after a new Moon. So as far as picking one to watch, it's one or the other. Barring inclement weather, the Perseids are this year's pick, especially if viewing can be done away from city lights. Caused by Earth's passage through the dust trail of Comet 109P/Swift-Tuttle, the Perseids usually produce up to 80 very swift meteors per hour as seen from sites away from city lights, peaking on the night of the 12th and extending through the predawn hours of the 13th, ideally during the hours between midnight and dawn.
However, don't limit your observing to just the night of the peak. According to the authoritative MeteorShowersOnline.com, "It is possible to spot five Perseids per hour at the beginning of August and perhaps 15 per hour by August 10. The Perseids rapidly increase to a peak of 50-80 meteors per hour by the night of August 12/13 and then rapidly decline to about 10 per hour by August 15." However, these estimates assume ideal circumstances and should be weighed against local observing conditions. From light-polluted urban areas such as San Francisco, for example, the number of visible meteors forecast is about 33 per hour at the shower's peak.
The September equinox occurs at 6:54 pm PDT on September 22. A popular misconception about this day is that day and night are exactly equal in length—or, expressed more accurately, that the Sun spends the same amount of time above the horizon as below.
Refraction of light by our atmosphere creates mirages that cause the top of the Sun's disk to appear above the horizon when it's actually below. This makes our star seem to rise sooner and set later than it really does, so "daytime" lasts a few minutes longer than "nighttime" (when the Sun is below the horizon).
The fact that the Sun is a measurable disk rather than a theoretical point also means that whether you define "sunrise" and "sunset" as when the top of the Sun's disk or its center are on the horizon also makes a difference. If you're wondering when the Sun really spends the same amount of time above and below the horizon, that's on September 26.
Visit an aquarium, planetarium, rainforest, and natural history museum—all under one living roof.
Download the Morrison Planetarium's 2018 Pocket Almanac to stay up-to-date on eclipses, meteor showers, satellite spottings, and more.
The Benjamin Dean lecture series brings the world's leading experts in astronomy, astrophysics, and more to the Academy's Morrison Planetarium.