Understanding Elongation in Astronomy: Find the Best Time to Observe Planets
In the Sky Tonight calendar, you can find such an event as “Greatest Elongation”. What does it mean, and why does it only happen to Mercury and Venus? What is common between the greatest elongation and the opposition, when planets and other celestial objects reach maximum brightness? Let’s see.
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What is elongation?
In astronomy, elongation is the angular distance between a planet (or other Solar System body) and the Sun as seen from the Earth. Elongations are measured in degrees. When we say that the object has an elongation of 90 degrees, it means it is located 90 degrees away from the Sun as observed from Earth.
As the brightest object in the sky, the Sun can obscure any other star or planet. Knowing the Sun’s position, you can choose the best time for observations (when the object you want to see is farthest from the Sun). Let's look at what elongation is, using the Solar System planets as an example.
Elongation: superior and inferior planets
The superior planets (planets that are farther from the Sun than the Earth), which include Mars, Jupiter, Saturn, Uranus, and Neptune, can reach any value of elongation from 0 to 180 degrees. For the inferior planets (planets that are closer to the Sun than the Earth), which include Mercury and Venus, the maximum elongation is about 28 degrees for Mercury and about 48 degrees for Venus. Some of the elongation values have specific names. Let’s go over them.
Conjunction
Solar conjunction is a celestial configuration in which the object has an elongation of 0 degrees, which means it is positioned at the same celestial longitude as the Sun, appearing very close to it in the sky. This alignment results in the object being obscured by sunlight, making it impossible to observe directly. Both superior and inferior planets can reach solar conjunction.
The terms “inferior solar conjunction” and “superior solar conjunction” are used in particular for Mercury and Venus. At inferior solar conjunction, the planet is between the Earth and the Sun. At superior solar conjunction, the planet is passing behind the Sun as seen from the Earth.
Quadrature
Quadrature is a position where the Sun, Earth, and the object form a right angle. As a result, the object has an elongation of 90°. This means that when the Sun sets, the object is reaching its highest point in the sky. A good example of quadrature is the First Quarter Moon, as this phase occurs when the Moon is at a 90-degree angle from the Sun. Quadrature is only possible for the superior planets.
Opposition
Opposition is a celestial alignment where the object has an elongation of 180 degrees, which is the greatest elongation for the superior planets. During opposition, the object and the Sun lie on opposite sides of the Earth, so the object is fully illuminated by sunlight and visible throughout the night. Opposition is considered to be the best time for observing a celestial body. In the dedicated article, we have a list of celestial bodies – planets, dwarf planets, and asteroids – that will reach opposition soon.
Greatest elongation
The inferior planets, Venus and Mercury, can’t reach quadrature and opposition. The greatest elongation possible for them is about 28 degrees for Mercury and 48 degrees for Venus. At greatest elongation, they are farthest from the Sun in the sky, so the skywatchers get the best chance to spot the planets. At greatest eastern elongation, Mercury and Venus are visible as evening objects. At greatest western elongation, they are visible as morning objects. Here's a list of the upcoming greatest elongations for Venus and Mercury.
September 5: Mercury at greatest western elongation
On September 5, at 01:59 GMT, Mercury (mag -0.4) will appear farthest from the Sun in the sky: the apparent distance between the two bodies will be 18°6'. Find Mercury before sunrise in the constellation Leo.
November 16: Mercury at greatest eastern elongation
On November 16, at 07:59 GMT, Mercury (mag -0.3) will appear farthest from the Sun in the sky: the apparent distance between the two bodies will be 22°30'. Find Mercury after sunset in the constellation Ophiuchus.
December 25: Mercury at greatest western elongation
On December 25, at 01:59 GMT, Mercury (mag -0.4) will appear farthest from the Sun in the sky: the apparent distance between the two bodies will be 22°30'. Find Mercury before sunrise in the constellation Ophiuchus.
January 10, 2025: Venus at greatest eastern elongation
On January 10, at 03:59 GMT, Venus (mag -4.6) will appear farthest from the Sun in the sky: the apparent distance between the two bodies will be 47°12'. Find Venus after sunset in the constellation Aquarius.
Understanding elongation: to sum up
Elongation is the angular distance between a planet (or other Solar System body) and the Sun in the sky. It is measured in degrees. The superior planets (Mars, Jupiter, Saturn, Uranus, and Neptune) can reach any value of elongation from 0 to 180 degrees. The inferior planets (Venus and Mercury) can only reach elongation from 0 to about 28 degrees (for Mercury) or 48 degrees (for Venus). Knowing what elongation is and how it affects the visibility of planets can enhance your stargazing experience.
