Synodic Period Calculator: Planetary Conjunctions and Oppositions

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Select a preset to auto-fill the sidereal period, or choose Custom to enter your own.
Select a preset to auto-fill the sidereal period, or choose Custom to enter your own.
Choose whether to display the synodic period in days or decimal Earth years.
Synodic periodModerate synodic period
779.92

Time between successive conjunctions or oppositions of the two bodies

Unitdays
Synodic period (days)779.92
Synodic period (years)2.1353
Conjunctions per year0.468
Relative angular speed0.4616deg/day
Synodic period (days)779.92
Synodic period (years)2.1353

Synodic period: 779.92 days (2.135 years)

  • The two bodies align (conjunction or opposition) every 779.9 days, which is about 2.14 Earth years.
  • With fewer than one alignment per year, observers may wait years between successive events.
  • The faster body gains ground on the slower at 0.4616 degrees per day, completing a full 360-degree lap in exactly the synodic period.

Next stepTo find the next specific conjunction date, combine this period with a known reference event date and count forward in multiples of the synodic period.

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