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Interplanetary Low Tide

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see captionMay 4, 2000 -- On May 5, 2000, the five naked-eye planets -- Mercury, Venus, Mars, Jupiter and Saturn -- will cluster together on the far side of the Sun. Although the planets will not be in a perfectly straight line, their approximate alignment in a 25 degree-wide region of the sky has triggered speculation in some quarters that interplanetary tidal forces might be magnified, leading to extraordinary effects here on Earth. 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?

Fortunately, no.

Ocean and crustal tides on Earth will be indistinguishable from normal on May 5th. As a matter of fact, tidal forces from Jupiter and the other planets will actually be at a low ebb this week.

Right: On May 5, 2000, the Moon, Mercury, Venus, Mars, Jupiter and Saturn will converge in an area of the sky about 25 degrees across. Unfortunately, this spectacle will not be visible to sky watchers because of the blinding light of the Sun, which will appear to be right in the middle of this planetary get-together.

"Tides on Earth are dominated by the Sun and the Moon," explains professor George Lebo from the University of Florida Department of Astronomy. "The alignment of the five naked-eye planets just doesn't make a difference. The tidal forces we feel from all those planets put together are thousands of times less than lunar and solar tides. In fact, these forces will be even lower than usual this week because all of the planets are about as far away from us as they can get -- on the other side of the Sun."

see caption

Above: This graphic by astronomer and mathematician Frank Reed shows the relative magnitude of tidal stretching on Earth due to Venus, Jupiter and Mars as a function of time between 1990 and 2007. The effect of the other planets is negligible on this scale. The Sun and the Moon were not included in the calculation. They exert tides that are about 10,000 times stronger than the peak tides noted in this graph. Venus produces the strongest tides on Earth because it comes closest to our planet. Jupiter's tides are approximately 10 times weaker. In May 2000, the total planetary tidal force felt on Earth is actually below average. [more information from Frank Reed's planetary tides home page]

Watching the Tide Roll Away....

see captionTides on our planet are caused by the gravitational pull of the Moon and Sun. Earth's oceans "bulge out" because the Moon's gravity pulls a little harder on one side of our planet (the side closer to the Moon) than it does on the other. The Sun's gravity raises tides, too, but lunar tides are twice as big. When the Sun and the Moon are approximately aligned (as they are when the Moon is New or Full) we experience especially high tides called "Spring Tides." When there is a 90-degree angle between the two, we get lower tides called "Neap Tides."

Ocean tides on Earth can be as high as 12 meters (40 ft) depending on local geography and the alignment of the Moon and Sun. Earth has solid ground tides too, but they amount to less than 20 centimeters (about 8 inches).

Although the Moon is small compared to behemoths like the Sun and Jupiter, lunar tidal forces are dominant because the Moon is so nearby. Tidal stretching is proportional to the inverse cube of the distance to the source of gravity. (If you moved the Moon to twice its present distance, its tidal influence would decrease by a factor of 23=8). Thus, nearby objects -- even small ones like the Moon -- raise high tides, while distant giants like Jupiter don't produce much of an effect.

"The force of Newtonian gravity falls off as the square of the distance," explains Lebo, "but tidal forces decline as its cube. Tides are caused by gravity, so why aren't they the same? The reason is simply that tidal stretching comes from a difference in the gravitational pull felt on two sides of a body. In a 1/r2 gravitational field, the difference in gravity between any two points falls off as the cube of the distance, not the square."

Jupiter's Terrible Tides (Not!)

After the Sun and the Moon, Venus is the object in our solar system that produces the biggest tides on Earth. This is simply because the Earth comes closer to Venus than any other planet. Even when the two are separated by their minimum distance of 0.3 AU, which happens every year and a half on average, Venus increases the size of our ocean tides by less than 0.005 cm. (Note: An AU, or "astronomical unit", is the distance between the Earth and the Sun. 1 AU = 1.5 x 108 km).

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
Tides caused by the biggest planet in our solar system, Jupiter, are 10 times less than the ones we feel from Venus. Once again the reason involves distance. Although Jupiter is 388 times more massive than Venus, its closest approach to Earth is 14 times greater than our planet's minimum distance to Venus.

Right: 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.

That's not to say that Jupiter isn't a powerful source of tides in its vicinity. You just have to be nearby to feel them. The innermost of Jupiter's big moons, Io, experiences tidal forces nearly 20,000 times stronger than the forces we feel here on Earth due to the Moon. Solid tidal bulges on Io are about 100m high, taller than a 40-story building! This stretching triggers some of the most active volcanoes in the solar system. (See, e.g., Jupiter's Terrible Tides).

Fortunately, Jupiter will be 928 million km (577 million miles) away on May 5, 2000, so the Earth won't have to endure the sort of extraordinary tidal stretching that Io undergoes every day.

Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story.

So, if you're planning festivities for Cinco de Mayo on May 5, 2000, you can probably leave your earthquake gear in the closest. Tides won't be stretching our planet any more than usual when the planets get together behind the Sun. As an alternative, you may want to try catching the eta Aquarid meteor shower, a nice sky show expected to peak during the early morning hours of May 5th and 6th.

Web Links

5/5/2000: The Meteor Shower -see eta Aquarid meteors on May 5, 2000

Planets for Dessert -Three planets and the moon clustered together in the sky for a beautiful show on April 6, 2000.

Planetary Alignments in 2000 - from the Griffith Observatory

Do planetary alignments cause earthquakes? - find out at badastronomy.com

Frank Reed's Planetary Tide Calculations