A Report On and Question About Mercury

[jrkirkham]

Last year I tried to catch a picture of Mercury near both eastern and western companion elongations. In January I snapped a picture, but February was cloudy, so I missed it. I tried again during the year, but weather or my schedule always stopped me. This weekend I was able to grab this picture only a couple of days after eastern elongation. When I was filling in the details I noticed something that I didn't expect. I took my January 2021 and 2022 pictures within a couple of days of each other. The time of the pictures was within 5 minutes of each other. The Alt. and Az. were within a couple of degrees of each other. Saturn and Jupiter were in different places with regard to Mercury, but I could have almost used last year's picture. I know that comets are predictable. I also think that Venus shines brightest somewhere around Feb. 12 each year, but I could be wrong. Is it usual for planets to be located in similar portions of the sky at similar times of the year? I never really thought about it before.

[notFritzArgelander]

This is the “synodic period “ for an object.

[OzEclipse]

Hi Rob,@jrkirkham
The time of year of an elongation, conjunction or opposition of any planet is related to the ratio of the Earth's orbital period to the planet's period. When I took up astronomy in 1978, Jupiter was in Gemini. I remember seeing it near Praesepe in December of that year. It's orbital period is 12 years so as we approach my 48th year (2025), it is heading back there.

With inferior planets, the same basic rule applies except that it happens much more often.
Mercury has an orbital period of 88 days 4.14 times less than the Earth. So it will be at greatest elongations at similar times in consecutive years. The planets are constrained to being near the ecliptic with deviations due to orbital inclination. The Sun sets at the same place at a given time of year so the greatest elongation will be 18 or so degrees from that point along the ecliptic. In the two diagrams below, the path of Mercury near greatest elongation in 2021 and 2022.

As the years progress, the 4.14 ratio means the elongation will migrate away from January then in subsequent years, the repetitive altaz relationship will break down. By 2024, the multiple of that elongation will have migrated forward to March April.

Meanwhile another elongation that was around Oct-Nov in 2020 will be creeping forward through December toward January. It's a constantly migrating series.

Venus is not every February.
The orbit of Venus, 225 days is 0.61 Earth years. or Earth's period is 1.6 Venutian years. So the elongations flip by about 6 months but also slowly creep because the ratio is not exactly 1.5 or 2/3 depending which way you look at it.

Hope this helps more than it confuses.
Joe

[jrkirkham]

That was very helpful nd interesting, Joe. Thank you.