What Do Eclipses, Spring Tides and the Full Moon Have in Common? A Straight Line
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Three bodies lining up in space is a single configuration with a single name, and a surprising number of separate-looking phenomena are all just consequences of it.
What the arrangement is
The term describes three celestial bodies falling on a straight line, and in ordinary use those bodies are the Sun, the Earth and the Moon. Two arrangements qualify. The Moon can sit between the Sun and the Earth, which is new moon, or the Earth can sit between the Sun and the Moon, which is full moon. Both are the same geometric condition seen from different sides. The word comes from a Greek term meaning yoked together, and it is used more broadly in astronomy for any such alignment, including a planet lining up with the Sun and the Earth.
What follows from it
A single geometry produces a long list of familiar effects:
- •New moon and full moon, by definition
- •The largest tides of the month, when both pulls add together
- •Solar eclipses, when the alignment at new moon is close enough
- •Lunar eclipses, under the same condition at full moon
- •Slightly increased strain on the Earth from combined tidal forces
- •The brightest and the darkest night skies of the month
Why eclipses are not monthly
If the alignment happens twice a month, the obvious question is why eclipses do not, and the answer is that the alignment is rarely exact. The Moon's orbit is tilted by about five degrees relative to the Earth's path around the Sun, so at most new moons the shadow passes above or below the Earth entirely. Eclipses require the alignment to coincide with the Moon crossing the plane of the Earth's orbit, and those crossing points themselves move slowly around the sky over about eighteen and a half years. The result is eclipse seasons roughly twice a year rather than twice a month.
The wider use of the term
Astronomers apply the word beyond the Sun and Moon and the extensions are worth separating. A planet aligned with the Sun and the Earth is in one of two such configurations, called opposition when the Earth sits in the middle and conjunction when the Sun does, and opposition is when an outer planet is closest, brightest and visible all night. Moons of the outer planets line up with their parent and the Sun constantly, producing frequent eclipses and shadow transits that are observable from Earth. The term is also used loosely for any striking alignment of several planets in the sky.
The tides it produces
The tidal consequence is the one most people notice without knowing the cause. The Sun raises tides as well as the Moon, at rather under half the strength, and when all three bodies line up the two effects reinforce each other and produce the highest high tides and lowest low tides of the cycle. Those occur at both new and full moon, roughly every two weeks, and the name for them has nothing to do with the season. When the Sun and Moon are at right angles instead, the effects partly cancel and the range is smallest. Coastal flooding risk peaks when these large tides coincide with a storm.
The takeaway
Three bodies on a straight line is one configuration with one name, occurring at both new and full moon, and it produces the largest tides of the cycle because the solar and lunar pulls add. Eclipses need the same alignment plus the Moon crossing the plane of the Earth's orbit, which is why they come in seasons twice a year rather than twice a month.