Image of Sputnik 1 from Air and Space Museum

Notes for the Season features information on meteor showers, space exploration events, and astronomy-related anniversaries from January through March 2018.


                             Sunrise              Solar Noon           Sunset

January 1           7:25 am PST         12:13 pm PST        5:02 pm PST

February 1         7:14 am PST          12:23 pm PST       5:34 pm PST

March 1              6:41 pm PST         11:22 am PST        6:04 pm PST

(Times are for San Francisco, CA, and will vary slightly for other locations.)


Notes for Winter

The planet Saturn

Earth's orbit around the Sun is elliptical rather than being perfectly circular. This means that there's a point in the orbit that's closest to the Sun (perihelion) and another that's farthest away from it (aphelion), and the difference between the two is about 4 million miles. On January 2, Earth is at perihelion. If we're closer to the Sun right now, why is it wintertime? That's because the seasons are not the simple result of Earth's distance from the Sun, but rather are due to the tilt of the planet's axis, which affects the duration, angle, and intensity of sunlight on different parts of our planet's surface as it rotates.

Artist rendering of space junk orbiting Earth

The Moon's phase aside, the Quadrantid meteor shower is usually one of the year's better displays. Active January 1-5, this shower produces variable numbers of meteors, radiating from the region between the constellations Boȍtes the Herdsman and Draco the Dragon, where the defunct-and-departed Quadrans Muralis the Wall Quadrant was located. The meteor count usually reaches around 40 per hour on the 3rd, but this year's peak comes only two days after a full Moon, when the Moon is still about half as bright as when full. This will unfortunately wash many faint meteors from view.

Artist impression of Draco, or Draconid meteor shower

What is a "blue Moon"? Generally, it refers to something that's rare, but many people believe that the term specifically applies to the second full Moon to occur in a calendar month, such as those occurring this year in January and March. However, that meaning is based on a misinterpretation by a magazine writer who mistakenly defined a blue Moon that way in 1946. However, before that error was made, editions of the Maine Farmer's Almanac dating to 1819 defined a "blue Moon" as the third of four full Moons in a single season (which is even more rare than two-in-one-month). For this to happen, a full Moon would need to occur very soon after the beginning of a season so that three more lunar phase cycles can be completed before the end of the season. That doesn't happen again until May 2019. We should point out that "blue Moon" is just a name—the Moon doesn't really change color.


Well, actually...

Following forest fires and major volcanic eruptions, fine particles of ash injected into the atmosphere scatter light and have made the Moon appear bluish in color. The size of the ash particles relative to the wavelength of different colors of light determines the resulting color of the Moon (or even the Sun).

Diagram of the perseid meteor shower

One more full Moon fun-fact: With two full Moons each in January and March, no full Moon occurs in February. This is purely a matter of how natural events occasionally line up with our calendar, and are not necessarily reflective of some anomaly in the natural order. However, upon learning that this circumstance occurred in 1866, Santa Rosa-born cartoonist Robert Ripley of "Believe it or Not!" fame sensationalized it to imply that the Moon wasn't seen at all during the entire month. In 1929, he published a drawing captioned "The Month Without a Moon!," depicting a terrified couple looking around in a forest (causing one to wonder if they were supposed to find the Moon hiding behind a tree?). Of course, the caption should've read "The Month Without a Full Moon," since the Moon was certainly seen in all of its other phases. Ripley also added that this wouldn't happen again for another 2.5 million years. He was a little bit off about that, too—this will be the fifth such occurrence since he wrote about it in 1929.

Leonid meteor shower

By the way, the January 31 full Moon passes through Earth's shadow, turning a coppery, reddish color in a total lunar eclipse. At that time, the Moon will be centered over the western Pacific Ocean and visible along the Pacific Rim. For San Francisco, the partial eclipse (when the Moon enters the shadow) begins at 2:51 am PST, with totality (when the Moon is completely immersed in the shadow) at 4:51 am PST. Between those times, notice the curvature of Earth's shadow—an observation that is made during every lunar eclipse, no matter what direction sunlight is coming from. Greek scholars concluded thousands of years ago that this proves that Earth is a sphere, since that's the only shape that casts a round shadow in all directions.

Geminid meteor shower

The word "equinox" comes from the Latin aequinoctium, meaning "equal night." Are day and night really both twelve hours long on the equinox? Although the Sun is theoretically above and below the horizon for equal periods of time, it's never quite that simple. Refraction by Earth's atmosphere bends sunlight and causes a mirage of the Sun to appear above the horizon when the Sun is actually still below it. A similar effect occurs at sunset, making the image of the Sun persist after the Sun is really below the horizon. Put these together, and that means the Sun is visible for about six minutes longer than twelve hours. On top of that, the fact that the Sun is a measurable disk rather than a point affects our definition of "sunrise" and "sunset," depending on whether those terms refer to when the top of the Sun's disk is on the horizon or the center of the disk. This makes a further difference of several minutes, adding up to an 8-minute difference. For the northern mid-latitudes (around 40 degrees north latitude), the actual date that day and night are of equal length is slightly before the March equinox, or about March 17.


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Planetarium by Night

Planetarium by Night

Morrison Planetarium is open for Thursday NightLife events, featuring special shows and presentations.