Posted by “Without a wish, without a will,
I stood upon that silent hill
And stared into the sky until
My eyes were blind with stars and still
I stared into the sky.” -Ralph Hodgson
The next month — from May 5th to June 5th — brings three of the most spectacular astronomy sights possible on Earth back-to-back-to-back for skywatchers of all types, without telescopes, binoculars, or any special equipment. Tonight, May 5th, marks what’s come to be known as a Supermoon, or the largest, brightest full Moon of the year.
Image credit: Chris Kotsiopoulos at Earth Science Picture of the Day.
Not that you’ll notice, mind you, unless you’ve got both an incredible eye and an incredible memory. The full Moon is, by far, the brightest thing in the night sky, outshining the brightest star in the sky by a factor of around 40,000.
A supermoon, on the other hand, is only about 30% brighter than a normal full Moon.
Image credit: science @ NASA, retrieved from Tara Hastings at WDTN.
The Moon, of course, orbits the Earth in an ellipse, rather than in a perfect circle. When the Moon is farther away from Earth in its orbit — or closer to apogee — it appears smaller in the sky, while when it’s closer to Earth — near perigee — it appears larger. The supermoon is the one full Moon out of the year that occurs when the Moon is at its minimum distance from the Earth, and hence appears the brightest.
Image credit: Essay Web's astronomy site.
These differences, however, are relatively small. The full Moon at apogee is only 20% smaller than the full Moon at perigee, a difference completely un-noticeable to even a trained observer, unless you put these two images right next to one another.
Image credit: Marco Langbroek, the Netherlands, retrieved from surfersvillage.com.
Having the closest, brightest full Moon of the year is a great excuse to go out and look at it, attempt to photograph it, or if you’re far away enough from streetlights, enjoy the shadows cast by the moonlight.
There’s nothing you can’t do with a supermoon that you couldn’t do with any, ordinary full Moon, but it is fun to think about why this happens.
Image credit: Ryan, footnote #3 on Carrie Fitzgerald's site.
The Moon makes an ellipse around the Earth, which in turn makes an ellipse around the Sun. Right now, the Moon’s perigee is in the opposite direction of the Earth from the Sun, so full Moons appear as large as they’re ever going to. New Moons and crescents, on the other hand, will appear somewhat smaller, as they occur closer to apogee.
But six months from now, the Earth (and Moon) will be on the other side of the Sun, so the Moon’s apogee will occur close to the full phase, resulting in somewhat smaller full Moons, while new Moons and crescents will be larger. You can see NASA’s apogee and perigee calculator for more information, but I think the diagram below illustrates things pretty clearly.
Image credit: NASA, Fred Espenak and Jean Meeus.
Right now, we’re extremely close to position “C” in the diagram above, where the Moon’s apogee (farthest from Earth) occurs close to the Sun, and the Moon’s perigee (closest to Earth) occurs away from the Sun. This gives us the supermoon that you can see tonight, but fifteen days from now, it’s going to give us something far more rare and special.
The Moon’s apogee occurs on May 19th, and the very next day, at nearly its most distant from Earth, the Moon, Earth, and Sun will all line up, producing the spectacular and rare sight of an annular Solar Eclipse!
Image credit: Kopernik Observatory and Science Center.
On the evening of May 20th in North America, close to Sunset, the Moon will pass in front of the Sun. But because the Moon is so close to apogee, it will actually appear ever so slightly smaller than the Sun in the sky, and thus will not be sufficiently large to block it completely!
Astute skywatchers who plan their trip right and are blessed with clear skies will get to observe the elusive “Ring of Fire” shown above. I’ve already written my eclipse guide for those of you preparing to join me in watching this, but there is one cheap piece of equipment I’ll recommend that everyone pick up for looking at the Sun: a pair of Welder’s Goggles.
But there’s another reason to get welder’s goggles that’s even more rare and spectacular than the upcoming solar eclipse. Those of you who’ve had clear skies in the west for the past month or two may have noticed an extremely bright object there just after sunset.
Image made with stellarium.
This is what the night sky will look like at 9 PM at 45 degrees latitude (where I live) tonight. That bright object, fifteen times brighter than Sirius, is the planet Venus, which just achieved its greatest apparent brightness in the sky. (And appears as a gorgeous crescent with binoculars if you can focus properly!)
Venus, being an interior planet to Earth, appears brightest not when it is closest to us, nor when it’s in its full phase, but rather when it’s a crescent, where the combination of proximity to us and the amount it’s illuminated is maximized.
Image credit: Torquay Boy's Grammar School's Observatory.
Over the coming month, Venus will descend in the sky, with progressively less and less of the planet becoming illuminated to our eyes, percentage-wise. However, Venus’ angular size will increase, as the apparent diameter of the planet will increase in the sky due to it physically getting closer and closer to us.
Image credit: Shamefully retrieved from victoriastaffordapsychicinvestigation.wordpress.com.
Eight years ago, Venus didn’t just pass interior to Earth, missing the Sun by just a degree or two; in 2004, Venus actually transited across the disc of the Sun, blocking a small fraction of the Sun’s light. These transits are incredibly rare; you and I will get two in our lifetimes.
The last Venus transit before the 2004 one took place in 1882, and the next one won’t be until 2117. Unless, that is, you’re ready on June 5th of this year.
Image credit: Tonk at CloudyNights.
It is perfectly safe to look directly at the Sun for brief periods of time with a good pair of Welder’s goggles, and I’ve already got mine.
Where should you be to see it? That depends on where you live.
Image credit: Fred Espenak / NASA, retrieved from eclipse.gsfc.nasa.gov.
Where I am in North America, the Venus transit will start at about 3:00 PM on June 5th and will continue through sunset. The entire transit won’t be visible in North America, as it takes many hours to complete, but this is your one chance to witness an event like this with your own eyes.
In Europe, parts of Africa, and most of Asia, of course, you’ll be able to see the transit in the morning of June 6th instead. But those of you living in Iceland get the most special treat of all: a transit that spans both sunset and sunrise!
Three major astronomical events — the supermoon, tonight, the annular solar eclipse, on May 20th/21st, and the transit of Venus, on June 5th/6th — all occurring within a month of one another! There’s never been a better time to purchase a pair of welder’s goggles, that’s for sure!
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