Seeing Earth's Shadow and the Belt of Venus

Earth's Shadow and the Belt of Venus

While waiting for the sky to darken, especially when I am at a star party, I like to point out the many subtle changes in the night sky that unfold from the moment of sunset to the end of dusk. One of my favorites is the phenomenon known as the Belt of Venus, a beautiful pink and blue band that stretches along the horizon exactly opposite the point of sunset or sunrise.

Most of the time I see this phenomenon in the evening, since that is when I am most often focused on the sky while it transitions from light to dark and the star gazing begins. Of course, the reverse effect is plainly visible in the morning just before sunrise, and all of the elements of the effect move in reverse. Recently I had an unusual experience with the morning version of this phenomenon, and it was accented by the Moon.

In January, while driving to work early in the morning, I was particularly captivated by the setting Full Moon. The previous night was the exact time of the Full Moon, so as I left home shortly before sunrise, the bright Moon was just setting to my right in the clear western sky as drove south along the Great Highway in San Francisco, directly on the coast. During the 8 or 10 minutes I was driving along the coast, the Moon was quickly dropping low on the western horizon, minutes away from setting. I could clearly see the Belt of Venus emerge, as the dark shadow of Earth became more and more distinct low on the horizon. The specific thing that fascinated me was that the Moon itself, as it set, appeared to be “pulling” the band of darkness with it. The lower the Moon, the lower the dark shadow band on the horizon. This image from Jeff Sullivan captures the situation perfectly.

The technical underpinning for this sight was that a Full Moon is precisely opposite the Sun, and the Full Moon, a few hours after the exact moment of fullness, appears directly in the dark shadow band of the Earth and not above it – otherwise it would not be a Full Moon. And if this happened during an Eclipse Season, the Moon would be encapsulated by the distant shadow of the Earth and would itself be darkened, along with the atmosphere around its position in the sky. The morning I observed this in January was not at a time of eclipse, but nonetheless the Moon was decidedly well placed in the shadow band of Earth, and as it slipped below the horizon, the dark shadow set, and the pink belt faded, and minutes later the Sun peeked up above the horizon. What a lovely view that was, indeed, appearing as if the Moon was pulling the dark band with it, but in fact, the Moon was simply in lock step with the shadow, inseparable at that particular point in its ever-so-slightly-post full phase.

The subtle changes in the dusk and dawn sky are marvelous, but this phenomenon, the Belt of Venus, is particularly subtle because most people like to see the sunset or sunrise, and rarely turn around to look at the other horizon. Next time you get a chance, look, and you’ll be pleasantly surprised, particularly if you witness it at a Full Moon with a crystal clear horizon.

Here are two excellent articles about the Belt of Venus by astronomers I respect:
Tony Flanders of Sky & Telescope.
Phil Plait of Bad Astronomy on Slate

Image courtesy Wikimedia Commons.

KALW Broadcast: Asteroids, Comets and PanSTARRS

I paid a visit to KALW, San Francisco Public Radio, and talked with Ben Trefny about asteroids, comets and of course, PanSTARRS on their evening program Crosscurrents. Listen in to our lively discussion about the beauty and dangers of things roaring by from outer space! Click here to listen.

A Comet in the West: PanSTARRS from San Francisco

Comet Panstarrs in March

We have a comet about to emerge into the evening sky. PanSTARRS will be a nice sight, not a magnificent comet but an easy target that will be visible in the dusk sky in the next 2-3 weeks. I plan to look for it from Ocean Beach here in San Francisco, where the clear ocean horizon should provide a good setting.

Comet PanSTARRS is currently visible from the Southern Hemisphere, but will move north and into view later this week. Although its low apparition will not make it shine very brightly, it will be a lovely sight for those who make the effort to view it, especially through binoculars, where the image is best seen. A telescope will zoom in too far, and naked eye viewing of PanSTARRS will probably be unimpressive, so I strongly recommend a pair of binoculars to see the comet at its best.

Sky & Telescope Magazine has a very good updated page on the latest on the comet. And more information is also available on the Earth Sky website.

Image courtesy of Sky & Telescope.

Update March 12, 2013: Saw PanSTARRS from high above the Bay Area, on Skyline Boulevard near highway 92. I needed binoculars to see it well, making a beautiful appearance next to the young crescent Moon. It's work to find, but worth it.

KGO Broadcast - February 16, 2013

Last Saturday I had a fun experience: joining KGO 810 Talk Radio in San Francisco for a one-hour broadcast. Greg Gory invited me to stop by and talk about asteroids, meteors, fireballs and many other astronomy topics, and we had a fine time doing all of that and more! We entertained calls from listeners and ended the show on a high note, discussing why we go into space and explore in the first place. Click here to listen.

A Perfect Time to see Mercury

Mercury in February

Mercury is the most elusive of the visible planets in the Solar System, never drifting far from the glare of the Sun, and always on the move. So you should take advantage of the upcoming weeks when Mercury will be well positioned in the sunset sky. Mercury is a small planet so it will not have the bold bright shine of Venus or Jupiter, but it will certainly stand out as a bright speck of light in the dusk sky. Look soon after sunset for a point of light that becomes increasingly bright as darkness sets in. You'll need a good, clear western horizon to see this. But take a minute some evening and spot it, since it won't be this easy again for a while.

Image courtesy Sky & Telescope.

Asteroid Alert! 2012 DA14 swings by Earth

This Friday, a small asteroid will pass quite close to Earth. 2012 DA14 is a 60 meter ('Olympic pool size') asteroid that will pass between the low-Earth orbit satellites and geostationary satellites, at a distance of about 17,000 miles, a close shave by astronomical terms, but not something that is posing any kind of threat to the planet.

Asteroids pass near to Earth all the time, and the largest of them are being closely tracked. At this time over 1300 potentially hazardous asteroids (PHAs) are being watched, but none of them are on a collision course with Earth. For the foreseeable future, we have nothing to worry about - - at least as far as asteroids go!

If you want to watch the fly-by, you can check out a number of websites that will have live video feeds of the asteroid. At closest approach, it will be daytime in California and not visible, but the video feeds will be delivered from the other side of the planet. It won't be overly dramatic -- just a spec of light moving against the backdrop of stars. As one website put it, the size and distance of 2012 DA14 will be, on the scale of a 12-inch globe of the Earth, a grain of sand 2 feet away from the surface of the Earth. That's a good reference.

Get Involved: Star Parties/Learn to Use a Telescope

Every month there are numerous opportunities to get involved with the fun and science of astronomy. The NASA Night Sky Network is a good place to start if you want to find a local event, and in this post I'll provide some local suggestions if you live in the San Francisco Bay Area.

Star Party

I love presenting the sky at star parties and giving talks about astronomy, so I signed up to do this for the Marin County "One Book One Marin" celebration happening this year. Check the calendar for dates, and join me at different libraries in Marin for an introductory lecture on the Solar System and constellations, and star gazing (if weather permits). The first two events are on February 11th and 12th. The San Francisco Amateur Astronomers will also host star parties on different nights in February and March.

The San Francisco Amateur Astronomers (SFAA) club has an open monthly meeting, where anyone may attend to hear very topical lectures on a variety of subjects related to astronomy and space research. The upcoming lecture and calendar are found on the website. And on Saturday February 16th the SFAA is hosting a Star Party and New Scope Forum where you can dust off and bring out your old telescope for a primer on how to use it, and then a chance to put it to work looking into the heavens.

The Peninsula Astronomical Society and the always-excellent Dr. Andrew Fraknoi put on star parties and outstanding lectures in Los Altos Hills. And the wonderful Chabot Space & Science Center has very fine telescope nights featuring their giant telescopes on Friday and Saturday nights year-round.

Get involved! You won't be sorry.

Planetary Conjunction at Dusk: Mars and Mercury

Mars & Mercury

This week, you will have a chance to spot elusive Mercury in the dusk sky, as the fast-moving planet gradually slides into the evening sky for a February showcase. In the first few days of Mercury's evening apparition, it moves past Mars, which itself is fading into the evening twilight. To spot these two planets in conjunction (close encounter) on February 7th and 8th, you will need binoculars or a telescope, and a clear western horizon. The two will form a very close pair for a couple days, and then Mars will continue its fade into the glare of sunset while Mercury will race eastward, appearing higher in the sky for the middle of the month. It will be work to find these two, but a fun challenge if you have clear western skies.

The image shows the view 30 minutes after sunset. The Sun sets in San Francisco around 5:40 pm this week.

Here's a helpful short video clip from Sky & Telescope's Tony Flanders showing how to find the two planets in conjunction, and how to find Mercury during the month of February.

Image courtesy of Sky & Telescope.