Dear Friends,
For anyone North and South America and most of Europe and Africa, this Sunday night (Sept. 27) is a not-to-miss sky event: a total lunar eclipse. Better yet, for all of you in the continental U.S., this particular eclipse occurs in the evening — which means you can see most or all of the action before it’s time for bed, making it an especially great family viewing opportunity. Moreover, all you need to see it is a view of the Moon in the sky — if it’s not cloudy and you have a clear view toward the east (where the Moon rises), you don’t even have to leave home. Of course, it can be more fun to watch it with a group, and especially a group that includes people who can explain it to you, so I strongly suggest you check your local science center, planetarium, astronomy club, or university to see if they have anything planned. For example, those of you in my hometown of Boulder, CO, can go watch it at Fiske Planetarium. Here are the most important times for watching:
- The noticeable part of the eclipse begins when the Moon first enters Earth’s shadow. This will be at 9:07pm eastern time, which mean 8:07pm central, 7:07pm mountain, and 6:07 Pacific (but for the west coast, the moon will not yet have risen in the east).
- It’s worth watching from the above start on, but if you are time limited, the best part to watch is the onset of totality, which occurs at 10:11pm eastern (9:11 central, 8:11 mountain, 7:11 Pacific). Start watching at least about 15 minutes before totality, so you can see the change as the shadow moves across the face of the Moon. (Also note the curvature of the shadow, which is proof that Earth is round.)
- For more viewing details, there’s a nice article from Space.com that includes a great little video explanation, and also this great article from Sky and Telescope. You can find many other articles as well.
Now, about the subject line I used above… If you watch the news, you will hear a lot of terms being thrown around about this eclipse, which is said to feature a “super moon,” a “blood moon,” and a “harvest moon.” Here’s a brief overview of what these terms mean:
- The important part is the lunar eclipse, which occurs when the Moon passes through Earth’s shadow. If you think about it, you’ll realize that Earth’s shadow must point directly away from the Sun, which means that a lunar eclipse can occur only when the Moon is opposite the Sun in our sky — which means full moon. The reason we don’t get a lunar eclipse with every full moon is because the Moon’s orbit is slightly inclined (about 5°) to Earth’s, so most full moons are not quite perfectly aligned with the Sun and Earth (and therefore with Earth’s shadow). Alignments leading to a lunar eclipse usually happen about twice a year, so they are not especially rare — but not all lunar eclipses are total, and not all are visible from where you live, so this is still a spectacular opportunity on Sunday night. (Worth noting: a lunar eclipse can be seen from anywhere on the night side of Earth, since that means you can see the full moon. In contrast, a solar eclipse, which occurs when the Moon’s shadow falls on Earth during a new moon, is visible over only a small region of Earth — the reason being that the Moon is much smaller than Earth and therefore its shadow can cover only a small area of Earth at any one time.)
- The “super moon” designation applies to any full moon that happens to occur when the Moon is in the part of its orbit that brings it closest to Earth. The term is loosely defined, but you can probably see the idea: just like bringing your thumb closer to your eye makes it look bigger (in angular size), the Moon looks bigger if it is full when it is closer to Earth. The reason I said “sorta super” in the subject line is because the difference in angular size between the “super moon” and a “normal” full moon is fairly small (only a few percent) and therefore isn’t very noticeable to the eye.
- “Blood moon” is a term sometimes used to describe the Moon during a lunar eclipse, when the Moon turns noticeably red in color. However, it has absolutely nothing to do with blood, which is why I said “not so bloody” in the subject line. To understand the real reason why the Moon looks red during a lunar eclipse, consider the view of an observer on the eclipsed Moon: When looking at Earth, the observer on the Moon would see Earth’s night side (which will be blocking the view of the Sun behind it) surrounded by the reddish glow of all the sunrises and sunsets occurring on Earth at that moment; it is this reddish light that illuminates the Moon during the eclipse.
- The “harvest moon” is the name traditionally given to the full moon closest in time to the September equinox, which for this year occurs at 4:22am eastern time on Sept. 23 — which is tonight if you read this e-mail on the same night I sent it out. The harvest moon gets its name not only because it occurs in the traditional harvest season, but also because it is actually useful to farmers: Each night, the Moon rises later than it did the previous night, typically by about an hour. However, the precise amount later depends both on the time of year and your latitude, and it turns out that for a few days around the harvest full moon, the Moon rises only about half-an-hour later each night for northern mid-latitudes. This is useful to farmers because it means that for those few days, the light of the Moon is available around or shortly after sunset.
That should cover what you need for Sunday night. While I have your attention, a few other quick notes:
- There are just a few remaining shows in my “relativity tour” — click here to see where I’ll be or watch the video if you’ve missed it.
- Meanwhile, I’ve had so much fun doing the relativity tour that I’m starting a new “tour” for 2016: The “I’m not a Scientist” Guide to Global Warming. This presentation will premier at Fiske Planetarium on Oct. 29 and 30. If you are in Colorado, I hope you’ll come by. Tickets available here. I’ll also have a new web site for the tour that should be live in the next few days: www.globalwarmingprimer.com.
- My new children’s book, I, Humanity, is now available for pre-order. It will be launched to the International Space Station in December as part of the second book launch for Story Time From Space.
- And speaking of Story Time From Space, another book being launched is the huge “lost my name” hit from the UK which personalizes books to kids; they are running a contest in which you can enter a name to be selected for the book that will go to the International Space Station. You can enter the contest here.
As always, please feel free to reply with questions or comments and to forward this e-mail to others. You can subscribe or unsubscribe at https://www.jeffreybennett.com/blog/.
Hope everyone has a great time watching the eclipse!
Jeff