Will there ever be a time when all the planets of our solar system line up in a row, one behind the other, as seen from Earth? (continued)
Richard Swifte
Darmstadt, Germany
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An exact line-up would mean that each of our seven neighbouring planets eclipses those behind it, which, given their small angular sizes as seen from Earth, is incredibly unlikely.
Realistically, you should allow a lining-up to within at least a few degrees.
As an example, just considering longitude, Mercury (nearest the sun) laps Venus every 0.396 years. Allowing 1.8 degrees of separation gives an arc of 3.6 degrees, or one-hundredth of a circle. The chance of any other planet lining up within this arc is 1 in 100, so, on average, Earth will be in the arc every 39.6 years.
But actually, from Earth, we can see Mercury and Venus line up in four possible combinations (each planet on either side of the sun), so we can adopt about 10 years as the average line-up time. Adding in a further planet multiplies this period by 100. So a line-up of all the other seven planets as seen from Earth occurs every 100 billion years on average – much longer than the lifetime of the current stable solar system.
If we ignore Uranus and Neptune and just consider the planets visible with the naked eye, the average period is 10 million years. This makes it remarkable that, in 1993, within less than 4.5 degrees. This astronomical event may have been the starting point of the traditional Chinese calendar.
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