Conjunction and Opposition

Sometimes, there are unique occurrences when planets align themselves with the Earth and the Sun. We actually have specific names for these occurrences: opposition and conjunction.

 

Opposition occurs when the Sun and the planet are in opposite directions in the sky, i.e. 180° apart in the sky. The Full Moon is also an example of opposition. Venus and Mercury, however, can never be in opposition. In my post about the morning and evening star, we know that Venus is never farther than 47.8° and Mercury is never more than 27.8° from the Sun.

 

Conjunctions happen to all the planets. The best way to think of it is when a planet and the Sun are in relatively the same direction in the sky. When the outer planets (Mars, Jupiter, Saturn, Uranus, and Neptune) are in the same direction as the Sun, we just refer to it as a conjunction. However, since Venus and Mercury can actually be at conjunction at two different points in their orbits, we need to specify their two conjunctions. When the inner planet is closest to the Earth, we call that its inferior conjunction because it is closer to the Earth than the Sun and when its at its furthest point, we call that the superior conjunction.

Planets can also be at conjunction with other planets. A good example is now visible in the night sky. Right now for a few days, Jupiter and Venus are near each other in the sky, and depending on your telescope or binoculars, are in the same field of view.

Venus and Jupiter in conjunction, with the full moon to give scale.

Via APOD

Composite Image Credit & Copyright: Wang, Letian

 

 

Constellations and Asterisms

I've discussed constellations before but never really defined what they are. A constellation is a group of stars that are in the same approximate area in the sky. These stars generally are not associated with each other except that they are all in the same location in the sky.


For example, Orion has two really bright stars: Rigel, a blue supergiant that is the left foot of the Hunter, and Betelgeuse, a red supergiant that is the right shoulder. In the sky, they are only separated by 18.29°, but in space Rigel is 260 pc from Earth and Betelgeuse is 197 pc from Earth. Same area of the sky, but really far apart in space.

From APOD

Credit & Copyright: Matthew Spinelli

Betelgeuse is also part of something called an asterism: a smaller group of stars in the same general area in the sky that can either be a part of a constellation, or stars that are in other constellations. The asterism to which Betelgeuse belong is called the Winter Triangle along with Sirius (the Dog Star, the brightest star in Canis Major) and Procyon (the brightest star in Canis Minor).

Winter Triangle

Procyon, Betelgeuse and Sirius form the Winter Triangle.

Credit: StarryNightEducation

Remember the zodiac is made up of thirteen constellations. In all, there are 88 recognized constellations in the nighttime sky. I am not going to list them all here, but they can easily be found by using your favorite search engine to find them listed. Constellation names are used to actually describe stars in terms of what constellation you may find them. For example, α Centauri (one of the closest stars to our Sun) is found in the constellations Centaurus, hence Centauri. Rigel is also known as β Orionis and Betelgeuse is α Orionis. (I'll explain what the Greek letters before a constellation name means - Bayer designation).

Other example of asterisms include the Big Dipper and the Little Dipper, which are smaller portions of the constellations Ursa Major (Big Bear) and Ursa Minor (Little Bear), respectively. How many other asterisms do you know of?