Largest Satellites of Our Solar System

Here is a table of the 12 largest satellites in our solar system.  In addition to the size of each satellite, its home planet, its median distance from that planet, and discovery information, its median distance from its home planet is given in terms of the median lunar distance from the Earth.  Remarkably, Pluto’s moon Charon is just 0.05 lunar distances from Pluto, only 19,591 km.  Only one other of the largest satellites orbits closer to its home planet than the Moon orbits around the Earth, and that is Neptune’s moon Triton at 92% of the Earth-Moon distance.  At the other end of the scale, Saturn’s moon Iapetus orbits Saturn over nine times further away than the Moon orbits the Earth.

Now let’s look at the orbital eccentricity of each of the largest moons, and the orbital inclination relative to the equator of its home planet.

Our familiar Moon is really an oddball: it has the greatest orbital eccentricity of all the largest satellites, and, with the exception of Triton and Iapetus, by far the greatest orbital inclination relative to the equator of its home planet.  Triton is the oddball among oddballs as it is the only large satellite in our solar system that has a retrograde orbit: it orbits Neptune in a direction opposite the planet’s rotation.  Iapetus has an orbital inclination relative to Saturn’s equator almost as much as the Moon’s orbital inclination relative to the Earth’s equator, but this anomaly can perhaps be forgiven because Iapetus orbits so very far away from Saturn.  Its orbital period is over 79 days.

Note that the Moon’s orbital inclination relative to the equator of the Earth varies between 18.33˚ and 28.60˚.  This occurs because the intersection between the plane of the Moon’s orbit around the Earth and the plane of the Earth’s orbit around the Sun precesses westward, making an entire circuit every 18.6 years.

Ganymede
Titan
Callisto
Io
The Moon
Europa
Triton
Titania
Rhea
Oberon
Iapetus
Charon

Like Sun, Like Moon

The Earth orbits the Sun once every 365.256363 (mean solar) days relative to the distant stars.  The Earth’s orbital speed ranges from 18.2 miles per second at aphelion, around July 4th, to 18.8 miles per second at perihelion, around January 3rd.  In units we’re perhaps more familiar with, that’s 65,518 mph at aphelion and 67,741 mph at perihelion. That’s a difference of 2,223 miles per hour!

As we are on a spinning globe, the direction towards which the Earth is orbiting is different at different times of the day.  When the Sun crosses the celestial meridian, due south, at its highest point in the sky around noon (1:00 p.m. daylight time), the Earth is orbiting towards your right (west) as you are facing south. Since the Earth is orbiting towards the west, the Sun appears to move towards the east, relative to the background stars—if we could see them during the day.  Since there are 360° in a circle and the Earth orbits the Sun in 365.256363 days (therefore the Sun appears to go around the Earth once every 365.256363 days relative to the background stars), the Sun’s average angular velocity eastward relative to the background stars is 360°/365.256363 days = 0.9856° per day.

The constellations through which the Sun moves are called the zodiacal constellations, and historically the zodiac contained 12 constellations, the same number as the number of months in a year.  But Belgian astronomer Eugène Delporte (1882-1955) drew up the 88 constellation boundaries we use today, approved by the IAU in 1930, so now the Sun spends a few days each year in the non-zodiacal constellation Ophiuchus, the Serpent Bearer. Furthermore, because the Earth’s axis is precessing, the calendar dates during which the Sun is in a particular zodiacal constellation is gradually getting later.

Astrologically, each zodiacal constellation has a width of 30° (360° / 12 constellations = 30° per constellation).  But, of course, the constellations are different sizes and shapes, so astronomically the number of days the Sun spends in each constellation varies. Here is the situation at present.

Constellation
Description
Sun Travel Dates
Capricornus
Sea Goat
Jan 19 through Feb 16
Aquarius
Water Bearer
Feb 16 through Mar 12
Pisces
The Fish
Mar 12 through Apr 18
Aries
The Ram
Apr 18 through May 14
Taurus
The Bull
May 14 through Jun 21
Gemini
The Twins
Jun 21 through Jul 20
Cancer
The Crab
Jul 20 through Aug 10
Leo
The Lion
Aug 10 through Sep 16
Virgo
The Virgin
Sep 16 through Oct 31
Libra
The Scales
Oct 31 through Nov 23
Scorpius
The Scorpion
Nov 23 through Nov 29
Ophiuchus
Serpent Bearer
Nov 29 through Dec 18
Sagittarius
The Archer
Dec 18 through Jan 19

 

The apparent path the Sun takes across the sky relative to the background stars through these 13 constellations is called the ecliptic.  A little contemplation, aided perhaps by a drawing, will convince you that the ecliptic is also the plane of the Earth’s orbit around the Sun.  The Moon never strays very far from the ecliptic in our sky, since its orbital plane around the Earth is inclined at a modest angle of 5.16° relative to the Earth’s orbital plane around the Sun.  But, relative to the Earth’s equatorial plane, the inclination of the Moon’s orbit varies between 18.28° and 28.60° over 18.6 years as the line of intersection between the Moon’s orbital plane and the ecliptic plane precesses westward along the ecliptic due to the gravitational tug of war the Earth and the Sun exert on the Moon as it moves through space.  This steep inclination to the equatorial plane is very unusual for such a large moon.  In fact, all four satellites in our solar system that are larger than our Moon (Ganymede, Titan, Callisto, and Io) and the one that is slightly smaller (Europa) all orbit in a plane that is inclined less than 1/2° from the equatorial plane of their host planet (Jupiter and Saturn).

Since the Moon is never farther than 5.16° from the ecliptic, its apparent motion through our sky as it orbits the Earth mimics that of the Sun, only the Moon’s angular speed is over 13 times faster, completing its circuit of the sky every 27.321662 days, relative to the distant stars.  Thus the Moon moves a little over 13° eastward every day, or about 1/2° per hour.  Since the angular diameter of the Moon is also about 1/2°, we can easily remember that the Moon moves its own diameter eastward relative to the stars every hour.  Of course, superimposed on this motion is the 27-times-faster-yet motion of the Moon and stars westward as the Earth rotates towards the east.

Now, take a look at the following table and see how the Moon’s motion mimics that of the Sun throughout the month, and throughout the year.

 
——— Moon’s Phase and Path ———
Date
Sun’s Path
New
FQ
Full
LQ
Mar 20
EQ
EQ
High
EQ
Low
Jun 21
High
High
EQ
Low
EQ
Sep 22
EQ
EQ
Low
EQ
High
Dec 21
Low
Low
EQ
High
EQ

 

New = New Moon
near the Sun
FQ = First Quarter
90° east of the Sun
Full = Full Moon
180°, opposite the Sun
LQ = Last Quarter
90° west of the Sun

 

EQ
= crosses the celestial equator heading north
High
= rides high (north) across the sky
EQ
= crosses the celestial equator heading south
Low
= rides low (south) across the sky

 

So, if you aren’t already doing so, take note of how the Moon moves across the sky at different phases and times of the year.  For example, notice how the full moon (nearest the summer solstice) on June 27/28 rides low in the south across the sky.  You’ll note the entry for the “Jun 21” row and “Full” column is “Low”.  And, the Sun entry for that date is “High”.  See, it works!