Astronomy Essentials

Greatest elongation, superior and inferior conjunctions: Definition for stargazers

Sun at center with one ring around for inner planets and one for Earth showing greatest elongation.
At superior conjunction, Venus or Mercury is behind the sun from Earth. At inferior conjunction, Venus or Mercury is between the Earth and sun. At greatest elongation, Venus or Mercury is most to one side of the sun. Around greatest elongation, these inner planets, Mercury and Venus, are at their greatest distances from the sun on our sky’s dome. Illustration via John Jardine Goss.

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 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.

Chart comparing Mercury apparitions in 2022.
View larger. | Mercury elongations in 2022, compared. Gray areas represent evening apparitions (eastward elongation). Blue areas represent morning apparitions (westward elongation). The top figures are the maximum elongations, reached at the top dates shown beneath. Curves show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40° north (thick line) and 35° south (thin). Maxima are reached at the parenthesized dates below (40° north bold). Chart via Guy Ottewell’s 2022 Astronomical Calendar. Used with permission.
Chart showing a maximum height of Venus at greatest elongation in March 2022 of about 25 degrees above the dawn horizon, for the Northern Hemisphere.
View larger. | The 2022 morning elongation of Venus, the brightest planet, will come on March 20. For the Northern Hemisphere, the low angle of the ecliptic – path of the sun, moon and planets – on spring mornings will keep Venus relatively low in the eastern predawn sky. Chart via Guy Ottewell’s 2022 Astronomical Calendar. Used with permission.
Chart showing a maximum height of Venus at greatest elongation in March 2022 of about 45 degrees above the dawn horizon, for the Southern Hemisphere.
View larger. | For the Southern Hemisphere, the March 20, 2022 greatest elongation of Venus will place this dazzlingly bright planet high in the eastern predawn sky. That’s because March signals autumn for the Southern Hemisphere. And the autumn angle of the ecliptic – path of the sun, moon and planets – is steep relative to the horizon on autumn mornings. Chart via Guy Ottewell’s 2022 Astronomical Calendar. Used with permission.

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, 2022
 in 
Astronomy Essentials

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