Astronomy Essentials

# Greatest elongation, superior and inferior conjunction

### What is an elongation?

Astronomers use the word elongation to describe the angular distance – the distance on the sky’s dome – between the sun and one of the inner planets in our solar system, Mercury or Venus. Elongations are measured in degrees eastward or westward of the sun. Greatest elongations signal the best time to observe one of the inner planets. At greatest elongation, Venus or Mercury are typically farthest from the sun’s glare.

At greatest eastern elongation, Mercury or Venus are visible as an evening object that sets in the west after the sun.

At greatest western elongation, Mercury or Venus are visible as a morning object that rises in the east before the sun.

Maximum distance of Mercury from the sun at greatest elongation = 28 degrees.

Minimum distance of Mercury from the sun at greatest elongation = 18 degrees.

Maximum distance of Venus from the sun at greatest elongation = 47 degrees.

Minimum distance of Venus from the sun at greatest elongation = 45 degrees.

### What is superior conjunction? And inferior conjunction?

At superior conjunction, Venus or Mercury are behind the sun from Earth.

At inferior conjunction, Venus or Mercury are between the Earth and sun.

### Elongation comparison for Venus

Not all of Venus’ greatest elongations are created equal. That’s because the farthest from the sun that Venus can ever appear on the sky’s dome is about 47.3 degrees. On the other hand, the least distance is around 45.4 degrees.

Elongations are also higher or lower depending on the time of year they occur and your location on Earth.

### Elongation comparison for Mercury

Not all of Mercury’s greatest elongations, however, are created equal. In fact, some are greater than others. That’s because the farthest from the sun that Mercury can ever appear on the sky’s dome is about 28 degrees. On the other hand, the least distance is around 18 degrees.

Also, elongations are better or worse depending on the time of year they occur and your location on Earth.

Bottom line: Greatest elongations of Mercury and Venus are the best times to observe these inner worlds, in either the morning or evening sky.

Posted
January 1, 2024
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Astronomy Essentials