Why opposition dates slide from year to year
Opposition is when the Sun, Earth and an outer planet line up, and it is when the planet looks biggest and brightest. Oppositions do not come back in the same month each year; they slide by the synodic period. This page carries a table of actual 2016-2040 opposition dates for Mars, Jupiter and Saturn, so picking a start date looks up the real dates that follow. Mars, for example, goes from January 16, 2025 to February 19, 2027, then March 25, 2029 and May 4, 2031 (UTC dates).
Why adding a mean period drifts
Mars has a mean synodic period of about 779.9 days, but its orbit is noticeably eccentric, so the real gap between oppositions runs anywhere from 764 to 811 days. Stepping 779.9 days from January 16, 2025 puts the 2033 opposition on August 2, more than a month after the real June 28, and over more cycles the error reaches about ±50 days. So the page uses actual dates inside the table, and outside it estimates from the mean synodic period (Mars 779.94, Jupiter 398.88, Saturn 378.09 days) and shows the expected error (Mars ±50, Jupiter ±13, Saturn ±9 days) as of October 2026. In "Enter my own" mode the error is unknown in advance, because it depends on the eccentricity of that orbit, so the page only notes that the dates are mean-period estimates to check against an ephemeris.
What this page does not do
Table dates are opposition instants computed from VSOP87 planetary theory and given as UTC dates, so in U.S. time zones an opposition late in the UTC day can fall on the previous evening. Altitude, apparent size and viewing hours are not covered. For Mars, closest approach can differ from opposition by several days. Mercury and Venus orbit inside Earth's orbit and never reach opposition.
Frequently Asked Questions
Its synodic period is about two years and two months (779.9 days on average), so each opposition comes a month or two later, and the eccentric orbit makes the gap vary from 764 to 811 days. That is why this page uses a table of actual dates.
Stepping the mean period up to 12 cycles from a real opposition can miss by up to about ±50 days for Mars, ±13 days for Jupiter and ±9 days for Saturn. Estimated rows beyond the table show that error; "Enter my own" results carry a general note instead, since the error depends on the orbit's eccentricity.
No. Both orbit inside Earth's orbit and never reach opposition, so they do not fit this calculation.