There is only one Jupiter, so why are there two sets of rotation longitudes in the observation records?
The equatorial cloud belts and high-latitude cloud belts do not rotate exactly the same and cannot be described by a "fixed surface" clock.
Planets and Small Solar System Objects
The streaks in photos of Jupiter come from the atmosphere. Cloud belts move differently at different latitudes, and observers use System I and System II to record two commonly used rotation rhythms. We put a mark on each of the two strips and compare how they pass through the center.
Guess first
Do the two sets of rotational longitudes mean that two Jupiters were recorded?
Show answer and explanation
No, it’s a rotation reference for different latitudinal zones. — The cloud belts of the same gas planet are not a single rigid surface.
Then connect experience and science
Let’s first think of Jupiter as a gas planet with cloud belts
The pattern represents cloud bands, not a fixed surface map like Earth's landmasses.
Different latitudes can have different angular speeds
Number the two rotation rhythms separately
System I is commonly used in the equatorial zone, and System II is used in higher latitudes. Both longitude values can be meaningful at the same time.
W₁=17.710°+877.90003539°d；W₂=16.838°+870.27003539°d
Add the observation direction and light travel time
In addition to Jupiter's own rotation, you also need to know the direction in which the Earth is looking toward it, and the delay of light from it to here.
The geometric central meridian contains the distance light travel time term
The bright disc is slightly different from the full disc
Phase correction moves the geometric central meridian to the center of the bright part. P then explains which way the North Pole axis tilts in the image.
Geometry ω → phase-corrected apparent central meridian
d is the number of days from the specified reference time, and the two W's use different rotation rates. First find the geometric central meridian, then add the light travel time and phase correction; the longitudes of systems I and II cannot be directly subtracted as celestial body movement.
give it a try
Fixed longitude markers, advancing time; note that the geometric central meridian and the bright part central meridian also contain a phase difference.
No need to memorize symbols
- System I/II
- Two sets of rotational longitudes used in the equatorial zone and higher latitudes
- ωI / ωII
- geometric disk central meridian
- DE / DS / P
- Earth latitude / solar latitude / north pole position angle
Algorithm, accuracy and scope of application
Main routes to achieve high precision in Chapter 43: date ecliptic VSOP87 Earth/Jupiter, longitude light travel time approximation in the book, rotation angle of the two systems, DE/DS, geometric central meridian and phase corrected bright disk central meridian, light aberration and nutation P. The low-precision geometry of Chapter 44 is not passed off as the high-precision method of this lesson; this lesson uses multiple dates of compiled AA+ for independent comparison. The two cloud band markers on the map respectively use the System I/II geometric central meridian projection, which is not the actual location of the Great Red Spot, and does not simulate cloud drift or fluid dynamics. The low-precision route of Chapter 43 has been used as the geometric calculation required for the low-precision satellite of Chapter 44. The low-precision central meridian and phase simplified formulas have also been implemented and verified in math, but are not displayed separately.
my understanding check
Are the markings on the two strips in this lesson a real forecast of the Great Red Spot?
no — Actual clouds will also drift, so the schematic markers are only used to tell two sets of longitudes.
What should you confirm before comparing two longitudes?
Are they using the same system and the same time? — I/II and geometric/visual central meridian cannot be mixed.
Jupiter itself rotates so fast, does the time of light still matter?
important — Fast-rotating planets will significantly change their orientation within tens of minutes of light travel.
First, it is necessary to explain which rotation system, which time, and geometry or the center of the bright disk, so that the longitudes can be compared.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 43 chapter
Planets and Small Solar System Objects · All interactive courses