What Is 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.
Contents
- What is elongation?
- Superior and inferior planets’ configurations
- Conjunction
- Quadrature
- Opposition
Greatest elongation: upcoming events in 2025
- March 8: Mercury at greatest eastern elongation
- April 21: Mercury at greatest western elongation
- June 1: Venus at greatest western elongation
- July 4: Mercury at greatest eastern elongation
- August 19: Mercury at greatest western elongation
- October 29: Mercury at greatest eastern elongation
- December 7: Mercury at greatest western elongation
- Elongation and phases of the planets
- Understanding elongation: to sum up
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.
Superior and inferior planets’ configurations
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: upcoming events in 2025
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.
March 8: Mercury at greatest eastern elongation
On March 8, at 05:59 GMT, Mercury will be 18° 12′ east of the Sun in the sky. The planet will be visible in the evening sky in the constellation Pisces. Mercury will have a magnitude of -0.4, visible to the naked eye. Additionally, bright Venus (mag -4.6) will shine in the same constellation.
April 21: Mercury at greatest western elongation
On April 21, at 18:59 GMT, Mercury (mag 0.3) will be 27° 24′ west of the Sun in the sky. This is Mercury's farthest elongation of the year! In addition, Mercury will form a beautiful planetary triangle with Venus (mag -4.7) and Saturn (mag 1.2) in the constellation Pisces. Look for the planets on the eastern horizon before sunrise.
June 1: Venus at greatest western elongation
On June 1, at 01:59 GMT, Venus (mag -4.4) will be 45° 54′ west of the Sun in the sky. The planet will shine brightly as the “morning star” for a few hours before sunrise in the constellation Pisces.
July 4: Mercury at greatest eastern elongation
On July 4, at 03:59 GMT, Mercury (mag 0.5) will be at 25° 54′ east of the Sun in the sky. The planet will be visible in the constellation Cancer after sunset. The Beehive star cluster will also be shining in the vicinity, and you will be able to observe the objects together with a pair of binoculars.
August 19: Mercury at greatest western elongation
August 19, at 09:59 GMT, Mercury (mag -0.1) will be at 18° 36′ west of the Sun in the sky. Observe the planet in the constellation Cancer before sunrise.
October 29: Mercury at greatest eastern elongation
On October 29, at 21:59 GMT, Mercury (mag -0.2) will be at 23° 54′ east of the Sun in the sky. The planet will be visible in the constellation Cancer after sunset. Additionally, reddish Mars (mag 1.5) will shine in the neighboring constellation Libra.
December 7: Mercury at greatest western elongation
On December 7, at 20:59 GMT, Mercury (mag -0.5) will be at 20° 42′ west of the Sun in the sky. The planet will be visible in the constellation Libra before sunrise.
Elongation and phases of the planets
Just like the Moon, planets have phases. Although in space, half of a planet is always illuminated by the Sun, the portion of the illuminated side that we can see from Earth changes depending on the planet's elongation. These phases can be observed through a telescope, and Venus, the brightest planet, can even show its phases when viewed through binoculars.
Phases of the inferior planets
As shown in the image above, inferior planets go through a full cycle of phases: from new phase at inferior conjunction to quarter phases at greatest elongations and full phase at superior conjunction. However, observing the full and new phases is extremely challenging because during these phases the planets are lost in the Sun's glare. Thus, the visible phases range from crescent to gibbous.
Phases of the superior planets
Superior planets do not display a full range of phases, but only subtle transitions between gibbous and full phases. They appear fully illuminated during oppositions, which is the best time to observe them.
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. Planetary phases and visibility depend on elongation, which is especially noticeable for inferior planets (Venus and Mercury). Want to find out when Venus or Mercury rises in your location? Use the free Sky Tonight app. To learn the best times for viewing the superior planets, check out our article on opposition in astronomy.
