0

The moon moves along the zodiac circle along with other planets, this is a known fact. But at the same time, its orbit is inclined to the plane of the ecliptic. According to the above diagram, the moon can be in the zodiac circle only during the passage of the lunar nodes. How is this possible?

enter image description here

Vladimir Orlov
  • 481
  • 4
  • 8
  • 1
    That diagram exaggerates the inclination of the Moon's orbit. The 5° inclination, combined with the Moon's distance make it much less significant than the diagrams shows. – Greg Miller Jul 25 '23 at 13:49

1 Answers1

7

The Zodiac isn't a circle, it's a band centred on the ecliptic. Traditionally, it extends ~8° north and south of the ecliptic. The inclination of the Moon's orbit is ~5.145°, so it's always within the Zodiac band.

From Wikipedia:

The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south (as measured in celestial latitude) of the ecliptic, which is the apparent path of the Sun across the celestial sphere over the course of the year. The orbital paths of the Moon and major planets are within the belt of the zodiac.

Here's a table from JPL which lists the inclinations of the planets' orbits (in decimal degrees). See Keplerian Elements and Rates, Table 1.

Planet Inclination
Mercury 7.00497902
Venus 3.39467605
EM Bary -0.00001531
Mars 1.84969142
Jupiter 1.30439695
Saturn 2.48599187
Uranus 0.77263783
Neptune 1.77004347

Mercury has the largest inclination, ~7°. All the other planets are within a few degrees of the ecliptic.

However, as Mike G mentions, that only tells us the maximum heliocentric deviation of the planets from the ecliptic plane, that is, as seen by a theoretical observer at the centre of the Sun. The maximum deviation observed from Earth is a little different. For example, here's a plot of the ecliptic longitude and latitude of Venus, courtesy of Horizons. The plot uses a 3 day time step. There are 5 steps (so 10 days) between numeric labels. Label #17 is 2023-Aug-16, #18 is 2023-Aug-26.

Venus ecliptic lon & lat, 2023

Here's a plot of the ecliptic latitude of Venus, spanning 8 years, which equals 13 sidereal periods (and hence 5 synodic periods) of Venus. The time step is 10 days.

Venus lat, 2016-2024

And here's the Moon's ecliptic latitude for this year, using a 1 day time step.

Moon lat, 2023

PM 2Ring
  • 13,836
  • 2
  • 40
  • 58
  • in the table you refer to are arcseconds, not degrees. "Traditionally, it extends ~8° north and south of the ecliptic."I don't know the exact value of the width, but I'm sure it can't be 16°. – Vladimir Orlov Jul 25 '23 at 08:13
  • 1
    @Vladimir No, the inclinations are given in degrees, and the rate of change of the inclination is given in degrees per Julian century. – PM 2Ring Jul 25 '23 at 08:35
  • Venus's ecliptic latitude as seen from Earth can approach ±8° when they are on the same side of the Sun. – Mike G Jul 25 '23 at 12:02
  • @MikeG Good point. I'll update my answer. – PM 2Ring Jul 25 '23 at 12:31
  • FWIW, for the last several weeks I've been observing Venus in the late afternoons (I'm in the southern hemisphere), so I have no excuse for not mentioning earlier that it's heading for a latitude minimum. ;) – PM 2Ring Jul 25 '23 at 13:03