Astro Bob blog: I'm definitely pro-crepuscular. How 'bout you?Sunbeams for everyone plus the Summer Triangle returns in the east during June evenings.
By: Bob King, Duluth News Tribune
I'm definitely pro-crepuscular. How 'bout you?
Beams of sunlight cross the sky from west to east and appear to converge over the mountaintops and the head of the photographer in this photo shot two days ago near Bishop, Cal., by Andrew Kirk.
Look at those beautiful anticrepuscular rays! And I do mean anti. Crespuscular rays, which most of us call sunbeams, are rays of sunlight that pierce through holes in clouds or radiate from around the sun when its blinding disk is hidden behind a cloud's edge. They look like spotlights or crowns of light and imbue the skyscape with a sense of grandeur.
Even the garden variety crepuscular rays are beautiful. The name comes from the Latin for "twilight" and refers to the most frequent occurence of the rays during early evening and morning hours. Photo: Bob King
Dust and water vapor in the air are the reason we see crepuscular rays. The particles reflect sunlight and render its beams in bold outlines against a deep blue sky. Bright rays alternate with columns of shadow created by other clouds that block the sunlight. Anticrepuscular rays are the identical phenomenon except they occur near sunset or sunrise, and the beams cross the entire sky appearing converging at a point directly opposite the sun. The convergence is simply a perspective effect; the rays are as parallel as railroad tracks but like those tracks they seem to touch in the distance -- a familiar optical illusion. As with crepuscular rays, anticrepusculars are shafts of sunlight pouring through holes in the clouds.
The Summer Triangle rises stealthily in the east as twilight gives way to night in early June. Look between 10:30 and 11 p.m. to see the luminaries Vega, Deneb and Altair. Created with Stellarium
With the new month, one of my very favorite star patterns is finally gaining ground in the eastern sky. You've probably met these characters before -- Vega, Deneb and Altair. They form one of summer's most easily-recognized asterisms called the Summer Triangle. Vega and Altair are relatively nearby with distances of 25 and 17 light years respectively. Deneb is an entirely different beast. 54,000 times more luminous than the sun and 108 times its diameter, you'd better stay out of its way. If you could move Deneb to where the sun is now, Mercury, Venus and Earth would all be orbiting inside its white-hot interior. The only thing diminishing the star's intrinsic intensity is its vast distance: 1425 light years separate us from Deneb's searing radiation.
Now this doesn't mean Vega and Altair are wimps -- they're also bright and bigger than the sun, but they stand out mostly because they're relatively close to Earth. If you could place Deneb at Vega's distance, it would shine 15 times brighter than Venus! Put Vega at Deneb's distance and you'd need binoculars to see it.
Can you find the trio of the Summer Triangle in this photo? Notice how prominently the Milky Way stands out in this region of the sky. A time exposure begins to reveal how many more stars pepper the Milky Way band compared to what we see with the unaided eye. Photo: Bob King
When the Summer Triangle returns, we're in for another treat. The bright band of the summer Milky Way courses right through the middle of the triangle on its way toward the southern horizon. As early as 10:30 you can see it in the east under dark skies, and the misty band only gets better the later you're out. Just to clear up any confusion. Every star you see belongs to our galaxy the Milky Way. The band itself, which is created by the stacking of many thousands of light years of ever more distant stars, stands apart from the brighter but less concentrated smattering of nearby stars found elsewhere in the sky. We see a distinct band, dense with stars, because the Milky Way is shaped like a flattened disk resembling a deep-dish pizza. Sun and Earth are embedded within the disk 2/3 the way from center to edge. When we look through the disk, the stars stack up across the light years to form a thick band. When we look above or below the disk, there are too few stars to create a band. Our gaze quickly takes us out of the galaxy and into the starless realms of intergalactic space. We'll spend more time exploring the Milky Way later this spring and summer.
The two sets of stars are no longer "twins". This photo was taken Tuesday; Mars is even closer to Regulus tonight. Photo: Bob King
One last item. Have you been watching Mars-Regulus versus Gemini's Castor-Pollux in the western sky these evenings? Mars and Regulus are getting closer and closer and really catch your attention. Look for them around 10:30-11 p.m. moderately high in the southwest in late twilight.