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 is typically farthest from the sun’s glare.
At greatest eastern elongation, Mercury or Venus is visible as an evening object that sets in the west after the sun.
At greatest western elongation, Mercury or Venus is 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? Inferior conjunction
Look again at the diagram at the top of this page.
At superior conjunction, Venus or Mercury is behind the sun from Earth.
At inferior conjunction, Venus or Mercury is between the Earth and sun.
Bottom line: Greatest elongations of Mercury and Venus are the best times to observe these inner worlds, in either the morning or evening sky.
Kelly Kizer Whitt has been a science writer specializing in astronomy for more than two decades. She began her career at Astronomy Magazine, and she has made regular contributions to AstronomyToday and the Sierra Club, among other outlets. Her children’s picture book, Solar System Forecast, was published in 2012. She has also written a young adult dystopian novel titled A Different Sky. When she is not reading or writing about astronomy and staring up at the stars, she enjoys traveling to the national parks, creating crossword puzzles, running, tennis, and paddleboarding. Kelly lives with her family in Wisconsin.
Like what you read? Subscribe and receive daily news delivered to your inbox.
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.