|
 March
30, 2000 -- Next Thursday, April 6, three planets and the
thin crescent Moon are going to put on a memorable sky show when
the quartet converge inside a circle just 9 degrees across.
"It's going to be beautiful," says Vince Huegele, a
researcher at the NASA/Marshall Space Flight Center. "And
best of all, you won't need a telescope to see it, just your
naked eye."
Right: Duane
Hilton's rendering of the April 6 planetary get-together
as seen above Bryce Canyon in Utah.
To admire the display, simply go outside after dinner on April
6 and look toward the southwest sky. Around 8 p.m. local daylight
savings time the slender crescent moon will be easy to spot about
30 degrees above the horizon. The brightest nearby "star"
will be Jupiter. At magnitude -2.1, the giant planet is 8 times
brighter than Saturn, which glows pale yellow less than 3 degrees
west of the Moon. Mars will lie a scant 1.1 degrees north of
Jupiter. The red planet (magnitude 1.4) will be about 3 times
fainter than Saturn (magnitude 0.3).
Although the planets and our Moon will appear to be close
together, there's no danger of a collision. While the Moon is
only 384,000 km away from us this week, Mars is 349 million km
away; Jupiter is 875 million km away; and Saturn is a whopping
1493 million km distant. When it comes to the sky, appearances
can be deceiving!
Jupiter, Mars, and Saturn will appear to be even closer together
on April 15 when the three will fit inside a circle less than
5 degrees across. That's the most compact grouping of any 3 planets
for the entire year. Nevertheless, the display on April 6 will
be more beautiful thanks to the delicate crescent Moon as it
passes by the trio.
But wait, there's even more: Cradled in the arms of the
crescent Moon will appear the ghostly outline of the full Moon,
a dim glow that astronomers call "Earthshine." Like
all the planets we see in the night sky, the Moon shines because
of reflected sunlight. The side of the Moon facing the sun shines
brightly, and the side facing away is nearly dark. The only significant
illumination on the "dark side of the Moon" is due
to Earthshine -- sunlight that bounces off the Earth and falls
on the lunar surface. A slender crescent Moon with Earthshine
is widely regarded as one of the most delicate and beautiful
sights in the night sky. It will be difficult to see from urban
areas, but should be easy to view from dark sky locations.
When Planets Align (or "Taking
Nostradamus to School")
 The
April 6 grouping of Mars, Jupiter and Saturn is just the prelude
to a grander alignment on May 5, 2000. On that date, the Moon,
Mercury, Venus, Mars, Jupiter and Saturn will all converge in
an area of the sky less than 25 degrees across. Unfortunately,
the spectacle will be lost in the glare of the Sun, which will
be right in the middle of this planetary get-together!
Although the May 5 alignment won't produce a visible sky show,
some people are touting the event. Why? According to doomsayers,
the alignment will cause earthquakes, floods, and maybe even
split our planet in two like a meat cleaver through a head of
lettuce! In all fairness that sounds like an event to note on
your calendar.
If these sensational claims sound familiar, it probably means
you were alive 38 years ago when there was a similar celestial
alignment. On February 4, 1962, the Sun, the Moon, and all the
planets from Mercury to Saturn were clustered within a 17-degree
area of the sky. To top it off there was a total eclipse of the
Sun! Doom seemed certain to many astrologers and students of
Nostradamus.
In the May 1962 issue of the Griffith Observer, astronomer
Robert Richardson described the scene in Los Angeles on February
4, 1962:
Weeks beforehand we began getting inquiries [at the Griffith
Observatory] from people wanting to know, "What was going
to happen? What does it mean?" ... Sunday, February 4 [the
crowd at the Observatory] must have been the largest since it
was opened to the public in 1935. By two o'clock the road leading
to and from the observatory was a solid mass of cars lined up
bumper-to-bumper for half a mile. One woman was weeping so badly
it was hard to understand her. She was practically on the verge
of collapse. "I know it's silly to carry on this way,' she
gasped between sobs, 'but I can't help myself." [ref]
Fortunately, nothing happened. There were no severe earthquakes,
no devastating floods, no mass destruction. With no planet-wide
disaster to spoil the day, millions of onlookers enjoyed the
solar eclipse.
History shows that planetary alignments are harmless. In fact,
alignments like the one in 1962 and in 2000 are fairly common.
The five naked-eye planets cluster together in the sky within
a circle 25 degrees or less in diameter once every 57 years,
on average. The next time it will happen is September 8, 2040.
The 2040 grouping will include Mercury, Venus, Mars, Jupiter,
Saturn and the crescent Moon. Clustered well to the east of the
Sun, the planets will stage a spectacular show at 7:30 p.m. in
the evening. (Mark your calendar now!)
If planetary alignments are common and the Earth is never
destroyed, why do predictions of doom seem to attend each one?
Many people believe that when planets are aligned their gravity
and tidal forces are magnified, leading to extraordinary effects
here on Earth. This seems reasonable. After all, "spring
tides" (peak ocean tides that arise bi-monthly) occur when
the Sun, the Moon and the Earth are nearly in a straight line
around the times of the New Moon and Full Moon. Shouldn't even
more powerful tides arise when lots of planets are lined up?
No.
Maximum Tidal Forces of the Sun, Moon, and Planets on the Earth
|
Solar System Object |
Tidal Force |
|
Moon |
2.1 |
|
Sun |
1.00 |
|
Venus |
0.000113 |
|
Jupiter |
0.0000131 |
|
Mars |
0.0000023 |
|
Mercury |
0.0000007 |
|
Saturn |
0.0000005 |
|
Uranus |
0.000000001 |
|
Neptune |
0.000000002 |
|
Pluto |
0.0000000000001 |
We can calculate the maximum tidal force that each planet
has on the Earth. These values are shown in the table. Tidal
effects depend on distance and mass--but especially distance.
Tides decrease in intensity as the cube of the distance
to the source of gravity. (If you move twice as far away from
a planet, its tidal force decreases by 23=8 times).
That's why the strongest Earthly tides are caused by the nearest
object -- the Moon. If all the planets were to align perfectly
with each one as close as possible to the Earth, their gravity
would raise the ocean tides by just one twenty-fifth of one millimeter.
Typical ocean tides on Earth caused by the Moon and Sun are thousands
of times larger than that. Clearly, the contribution of the planets
is entirely negligible, and it makes no difference to the Earth
whether they are aligned or not.
Above: In this table, adapted from the Griffith Observatory's
"Planetary
Alignments in 2000", the Sun exerts 1 unit of tidal
force on the Earth; the Moon has a little more than twice the
effect of the Sun; the other nine planets together with all their
moons add only another one five-thousandth as much.
So, if you've already packed away your Y2K survival gear,
don't worry. There's no planet-rending catastrophe in the offing.
Like most other 5ths of May, the one in the year 2000 is a more
appropriate occasion for Cinco de Mayo festivities than for disaster
planning.
For lesson plans and educational activities related to planetary
alignments, tune into the March 30 episode of Thursday's
Classroom. There is also a
version of this story for kids at NASA/Marshall's NASA
Kids web site. |
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