Why do conjunctions happen




















I queried the JPL Horizons database for all the dates that I was looking for, figuring I'd save myself time looking up these dates for future posts.

Famous last words. I spent all day playing with data tables and figuring out what they meant. If you're interested in Mars, here's an explainer on Mars' seasonal calendar ; I'll cover Mars, Venus, and Mercury orbital events in a future post. One warning: the dates in this post, particularly ones many decades in the past or future, are approximate.

Also, there are a variety of mathematical expressions and reference frames for the positions and orientations of the outer planets, and depending on which set you use, you could get answers that are different by a few days.

Some quick definitions: Opposition is when the Sun-Earth-planet angle is at a maximum, for planets beyond Earth; it's when the planet is opposite to the position of the Sun.

The angle would be degrees if all the planets' orbits were coplanar, but the orbits are all a little bit inclined with respect to each other.

Opposition also happens near the date that Earth is closest to the planet within one synodic period the time from one opposition to another. I usually think of opposition as being the best time to get photos of a planet because of the minimum distance, but although that may be true for Mars, relative proximity doesn't help you much with the giant planets.

They are so far away that the relative closeness doesn't dramatically affect how big they appear. What opposition does for giant planet observers is to improve the opportunity that observers have to see them.

When a planet is at opposition, it's up all night long, so you have a lot of dark sky time to observe it every night. Conjunction is when the Sun-Earth-planet angle is at a minimum, that is, when the Sun and planet are very close to each other in the sky. For one or two months around conjunction, it's impossible to observe an outer planet from Earth through conventional telescopes, and communications with spacecraft located at the planet are made difficult or impossible by the Sun's radio noise.

Planetary missions plan for this by curtailing their transmission requirements during these periods. A handy way to remember these first two terms is to remember that Earth is at the center.

When the Moon passes in front of the Sun, the result is a solar eclipse which is where the term ecliptic comes from. This occurs—somewhere in the world—every six months or so. How often do solar eclipses occur? Mercury and Venus can also pass in front of the Sun although much more rarely than the Moon , producing a transit of Mercury or a transit of Venus.

If the Moon passes directly in front of a planet or star, it is called an occultation. An occultation of Mars by the Moon, as seen from Singapore on the evening of April 17, Occultations can also be produced by planets. On October 1, , Venus will occult Regulus one of the brightest stars in the sky. On rare occasions, planets can pass in front of each other. If one planet partially obscures another, the event is known as a transit.

If one planet completely covers another, it is called an occultation. Hartigan notes that ancient great conjunction was not an especially close or remarkable one. After this great conjunction ends, stargazers need not wait centuries for the next close one.

Another rendezvous where the giant planets are separated by just six arc minutes will arrive on March 15, , Hartigan says. All in all, the great conjunction is a reminder of how one can find solace in the constancy of heavenly cycles over the millennia given the inconstancy of modern times, Faherty says.

Charles Q. Choi is a frequent contributor to Scientific American. In his spare time, he has traveled to all seven continents. Follow Charles Q. Visit the exhibition. The bright Venus and distant Mars are shown as Earth's heavenly neighbours in the west, during and after dusk, at a very cold temperature, about 10 degrees below zero, and above a snow-covered field.

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