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Planets for Dessert

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Artist's rendering of the conjunctionMarch 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")

diagram of alignment from 90 deg above the plane of the solar systemThe 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?


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.

Web Links

Planetary Alignments in 2000 - from the Griffith Observatory

Do planetary alignments cause earthquakes? - find out at