April 2004 Sky from the Keeble Observatory
Astronomy sometimes seems the hectic science. Ironically so, as the sky serenely
turns overhead, the daily and yearly patterns evoke a sense of calm. Yet, the news
and new information arrive almost in a flood at times, and these are such times.
In the past month we have learned more from our Mars rovers. Beyond the stark images
from the red, rubble-strewn landing sites, we now have firm evidence that the Opportunity
landing site, known as Meridiani Planum, was once under water. Mineral evidence
is compelling, but even stronger inferences are drawn from erosion patterns on layered
rocks themselves. What we seem to be looking at is sediment deposited at the shore
of a shallow, briny lake or sea. The erosion patterns suggest wave action, with
water flowing at perhaps several miles per hour. Investigators delayed nearly a
month in making the announcement of their findings, so that they could consult coastal
geologists not working on the NASA project.
Those enigmatic spherical deposits found by Opportunity, whimsically dubbed blueberries
- though they're not really blue, rather gray, but approximately the right size
for blueberries - have now been identified as being composed of the iron-rich mineral,
hematite. On Earth, hematite forms in the presence of standing water. Both rovers
will be sent on long traverses from their present locations. Opportunity will attempt
to reach a nearby crater, somewhat larger than the small "Eagle" crater
where it landed. Spirit will be directed towards the "Columbia Hills"
to examine a geologically (Areologically?) different regime.
Hubble Space Telescope unveiled its deepest yet view of the Universe. The "Ultra
Deep Field" represents an accumulated 11-day exposure with the high resolution
Survey Camera. Directed at an "empty" patch of sky no bigger than a grain
of sand held at arm's length, the image reveals galaxies that formed when the Universe
was less than a billion years old.
Observers using the venerable 5-meter telescope at Palomar Observatory have identified
the most distant object yet confirmed as part of our solar system. Dubbed Sedna,
after an Inuit deity, this newly found member of our Sun's family lies nearly twice
as far as Pluto. It's a little smaller than Pluto, a lot redder, and is nearing
its closest approach to the Sun on an extended elliptical orbit. Its orbital period
is over 10,000 years ... carrying it nearly 1000 times as far from the Sun as we
live! Adding yet another object to the catalog of "trans-Neptunian" objects
raises again the call for "downgrading" Pluto from its historical designation
as a planet. It seems to be just the largest and first discovered of an entire class.
Lunar phases for April: Full Moon on the 5th, at 4:03 pm EDT (don't forget to turn
your clocks ahead on Sunday!); Last Quarter on the 11th, at 8:46 am; New Moon on
the 19th, at 8:21 pm; First Quarter on the 28th, at 12:32 am.
April is still a pretty good month for evening planet watchers, though we lose Mercury
into the Sun's glare by mid-month. As the sky emerges in the evening twilight, Venus
will be the brightest object to the west. It will be lower in the sky than last
month, but you can watch it for several hours until it disappears into the horizon
clutter and haze. Mars is seen above and to the left of Venus, a pale orange in
contrast to the brilliant white of Venus. Venus is catching up to us in its orbit,
and will lie directly in front of the Sun in early June. (This rare event, known
as a "transit," will not be visible from central Virginia.) This month
will see Venus' waning crescent in a small telescope. Saturn is about 50 degrees
above the western horizon at sunset, its rings remain a spectacular sight in a small
telescope. Jupiter emerges from twilight high to the southeast. An imaginary line
through these four planets marks the plane of our solar system, which is slightly
tilted relative to the Moon's orbit around Earth. Nevertheless, you'll see the Moon
pass close to these planets as the month advances ... near Jupiter on the 2nd, passing
Venus and Mars on the 22nd and 23rd, Saturn on the 25th, and then back to Jupiter
on the 30th.
An overhead view about two hours after sunset finds nothing prominent at zenith
(the point in the sky directly overhead). The bright "twins" of Castor
and Pollux are now to the west of zenith, and Leo is to the south. Towards the north
we see the inverted dipper of Ursa Major almost at zenith. And, to the east, the
sky appears almost empty except for the bright stars Arcturus in Bootes, and Spice
to the southeast in Virgo. Orion bids adieu for another season, as it settles below
the western horizon. The Milky Way arcs low from north to southwest ... crossing
above the western horizon between Orion and Gemini. A few hours later, the plane
of our Galaxy will nearly match the horizon.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.