Three Planets Bunch Up in Twilight (Forwarded)

Sky & Telescope
Cambridge, Massachusetts

Contacts:
Alan M. MacRobert, Senior Editor
617-864-7360 x151

Marcy L. McCreary, VP Marketing & Business Dev.
617-864-7360 x143

Press Release: June 17, 2005

Three Planets Bunch Up in Twilight

Gaze low to the west into the deepening twilight for the next couple of
weeks, and three planets will await your view. One is bright; two are
fainter. You can follow them through their celestial gyrations as they
shift position day by day.

The direction to look is low in the west-northwest. The brightest light
shining there is Venus, the "Evening Star." Dimmer Saturn is closing in
on it from the upper left, while Mercury is closing in on it from the
lower right. These three planets will form a remarkably tight bunch from
June 24th to 27th, appearing close enough together to be covered by your
thumb at arm's length.

After that, Saturn gets lower every day and sinks out of sight, while
Venus and Mercury remain closely paired into the first week of July. The
crescent Moon hangs with Mercury and Venus on July 8th. Meanwhile, the
background stars Pollux and Castor add to the scene. Binoculars will
enhance the view.

Accompanying this release is a series of daily sky scenes that
illustrate what to look for in the fading twilight on any date. They can
also be played in sequence as a QuickTime movie to show the dance of the
planets throughout this period.

A fourth bright planet is also in evening view, far to the upper left of
the group of three. This is Jupiter, shining high in the southwest.
Aside from Venus, it's the brightest point of light in the sky. (In
fact, some people may confuse it with Venus until they realize they
should be looking much lower and to the right for the Evening Star.)

All these planets appear about the same distance away as you watch them
in the deepening dusk, but this is an illusion. Mercury is roughly 90
million miles away (its distance changes during the period illustrated);
Venus is about 140 million miles away, and Saturn is 930 million miles
-- about 10 times farther than Mercury. Jupiter is about 500 million
miles away.

Such big distances are better expressed by how long it takes light to
cross them. Mercury is about 8 light-minutes distant, Venus is about 12
light-minutes away, and Saturn is 85 light-minutes from us. Jupiter is
45 light-minutes distant. By comparison, the stars Pollux and Castor, in
the constellation Gemini, are 34 and 52 light-years away, respectively.

NASA's Comet Crash in the Sky

There's another reason to start paying attention to the western dusk. On
the night of July 3rd, NASA's Deep Impact probe will slam at 23,000
miles per hour into the icy, mountain-size nucleus of Comet Tempel 1.
Nothing of this event will be visible to the unaided eye (contrary to
over-optimistic claims), but at least you can see the place in the sky
where it happens.

Find Jupiter again high in the southwest. Off to its left, by somewhat
more than the width of your fist at arm's length, is the fainter star
Spica, in the constellation Virgo. The impact with the comet will happen
a couple of finger-widths above Spica.

The crash is set to happen around 10:52 p.m. Pacific Daylight Time July
3rd (1:52 a.m. Eastern Daylight Time on the morning of July 4th). At
that time, Jupiter, Spica, and the comet's position will be in good view
from the western United States, especially the Southwest.

Deep Impact's 800-pound projectile should blast a crater anywhere from
the size of a large house to the size of a stadium in the comet's 9-by-2
1/2-mile nucleus, while instruments aboard the main spacecraft watch
from a safe distance. Scientists hope this first-ever comet excavation
will provide information about comets' internal composition and structure.

Some of the world's biggest telescopes will also be watching in the
following hours, days, and weeks. Countless amateur telescope users will
be watching too. But even during and after impact the comet is expected
to be faint, and telescope users will need to use a detailed star chart
to locate it.

A finder chart showing sufficiently faint stars accompanies this
release. Suitable charts also appear in the June 2005 issue of Sky &
Telescope, the July-August 2005 issue of Night Sky, and online at
SkyandTelescope.com. Anyone who's not already familiar with how to use
star charts with a telescope will need to follow the beginner's
instructions in the online article.

If your evening sky is clear on July 6th or 7th, there's a much easier
way to find Comet Tempel 1 with your telescope. Just center Spica in
your lowest-power eyepiece, then let the sky drift by (turn off the
scope's tracking motor if it has one). Wait exactly 20 minutes, and
Comet Tempel 1 should be in the field of view.

Much more about the Deep Impact mission and its target comet appears in
the cover stories of the June 2005 Sky & Telescope.

Sky Publishing Corp. was founded in 1941 by Charles A. Federer Jr. and
Helen Spence Federer, the original editors of Sky & Telescope magazine.
The company's headquarters are in Cambridge, Massachusetts, near the
Harvard-Smithsonian Center for Astrophysics. In addition to Sky &
Telescope and SkyandTelescope.com, the company publishes Night Sky
magazine (a bimonthly for beginners with a Web site at NightSkyMag.com),
two annuals (Beautiful Universe and SkyWatch), as well as books, star
atlases, posters, prints, globes, and other fine astronomy products.

Note to Editors/Producers: This release is accompanied by
publication-quality illustrations and a broadcast-quality video; see
http://skyandtelescope.com/aboutsky/...cle_1528_1.asp

Related Articles:

* Watching Comet Tempel 1
http://skyandtelescope.com/observing...cle_1522_1.asp