What Is Elongation in Astronomy: Mercury & Venus Greatest Elongation 2025-2026 Dates
Greatest elongation isn’t just a complex astronomy term – it’s your best chance to spot Mercury or Venus in the sky! Mercury reaches its next greatest elongation on December 7, so don’t miss the opportunity. Let the Sky Tonight app guide you to the perfect viewing moments: download it now and explore the night sky like never before! Plus, dive into our article on elongation and uncover the celestial mechanics behind these events.
Contents
- What is elongation?
- Inferior and superior planets’ configurations
- Elongation and phases of the planets
- Elongation of planets: 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.
Inferior and superior 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.
Solar 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 of Mercury and Venus: upcoming events in 2025-2026
The inferior planets, Venus and Mercury, can’t be in quadrature and opposition because they always stay between the Earth and the Sun. 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.
December 7, 2025: 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.
For observers in the Northern Hemisphere, this is the best morning visibility of Mercury! In early December, the steep angle of the ecliptic to the horizon lets Mercury climb higher in the sky before sunrise, making it easier to spot. In the Southern Hemisphere, Mercury will stay lower in the dawn sky, but it will still be further from the Sun’s glare than during many other apparitions, so it’s still worth a look.
February 19, 2026: Mercury at greatest eastern elongation
On February 19, at 16:59 GMT, Mercury (mag -0.4) will be at 18°07′ east of the Sun. The planet will be visible in the constellation Aquarius after sunset.
As a bonus, a thin waxing crescent Moon, Venus, and Saturn will gather in the same area of the sky. The Moon will be only 5% illuminated, so it may be hard to spot, especially if it’s low above the horizon, but Venus (mag -3.9) and Saturn (mag 1.1) will be easily visible without any optics. It’s a great chance to catch the Moon and three planets together in a small patch of sky!
April 4, 2026: Mercury at greatest western elongation
On April 4, at 01:16 GMT, Mercury (mag 0.3) will be at 27°49′ west of the Sun. The planet will be visible in the constellation Aquarius before sunrise. This is the year’s greatest elongation of Mercury, so don’t miss your chance to see this elusive planet!
Two more planets, Mars (mag 1.2) and Saturn (mag 0.9), will rise near Mercury. These planets are usually bright enough to be observed without optics, but this time they will be very close to the rising Sun, lost in its glare.
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.
Elongation of planets: 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.
