**Sidereal**("sid-EER-ee-al")- Relative to the stars. This is as close to "absolute position" as you can get in a planetary system, where everything is moving relative to everything else. The stars are very much farther away, so appear (relatively) fixed with respect to the planets in the system.
**Solar**- Relative to the sun.

Sidereal "zero" points of the Yavin-YavinIV system, as chosen for this discussion. |

Note the "zero-degrees" hashes through the diagram of the system. For reference, we will call this "sidereal zero". At this start point, we assume the Massassi Temple is at the sidereal zero on Yavin IV's surface. (If you look very closely you can see the tiny blue mark.)

We know that Yavin IV has a day 24 hours long (1). We assume for simplicity's sake that this is the sidereal day. That is, Yavin IV rotates exactly 360 degrees in 24 hours, and this is measurable relative to the "fixed" stars. As we see later, this is different from its solar day.

We have also determined that Yavin IV orbits the gas giant Yavin once every 12¼ hours.

After one complete rotation (day), Yavin IV is about 30 minutes' travel shy of completing 2 orbits around the gas giant. |

Yavin orbits its sun in approximately 13 "standard years" (1). If we assume a standard year of 365 "standard" 24-hour days, this makes a Yavin year 4,745 "standard" (sidereal) days long. I.e. Yavin IV will rotate on its axis 4,745 times in a Yavin year, the same as 4,745 SY4Days.

The even-block of 49 SY4Days does not go evenly into a Yavin year. It goes 96 times, with 41 days left over. After 49 Yavin years (637 "standard years") this works out. On New Year's Day, Yavin IV and Yavin are once again lined up on sidereal-zero at the sidereal-zero point of Yavin's orbit about the sun, for the first time after 637 standard years.

A Sideral Yavin IV Day is not the same length as its solar day. The time span from actual
sunrise to actual sunrise is different. Here's why:

When 49 SY4Days have passed, the Yavin system has moved 49/4745 of the way around the sun.
This works out to about 3.7°. This means that relative to the "fixed" stars, the
angle at which the sun's light hits them has also changed by 3.7°. Thus, as the system
has moved, the boundaries between the light and dark sides of Yavin and Yavin IV have
appeared to "rotate" 3.7° around the bodies.

Shift in the sidereal position of dawn (the dark/light boundary) |

It works out that after 49 days, a given point on Yavin IV sees sunrise 14 minutes and 52 seconds later than it did 49 days ago. A Yavin IV Solar Day (Y4SolDay) is thus 24 hours and 18.2 seconds long.

Note also that as the Yavin system proceeds all the way around the sun in one year, the dark/light boundary has also "shifted" around Yavin and Yavin IV a complete 360°. With the orbital and rotational directions we are assuming, this means that Yavin IV "loses a day." That is, it sees 4744 sunrise-to-sunrise cycles in a sidereal year. Thus the Yavin sidereal year is 4745 sidereal days long, but 4744 solar days long. (By the way, we experience the same case here on Earth.)

We can also determine the period of a solar orbit of Yavin IV (Y4SolOrbit). This is the time it takes for Yavin IV to line up exactly with the dark/light boundary of Yavin, go around the gas giant once, and again line up exactly with the dark/light boundary, accounting for the fact that the system has moved in its orbit around the sun, causing the dark/light boundary to "rotate" slightly forward around Yavin relative to the sidereal stars. A Yavin IV solar orbit will thus be slightly more than 360°.

(Yavin IV orbital rate)x(t) = 360° + (dark/light mov't rate)x(t)

(360/12.25)x(t) = 360 + (360/113880)x(t)

Due to this extra time for a solar day, a given point on Yavin IV's surface does *not*
line exactly up again, with the dark/light boundary, (from dawn to dawn) in 49 SY4Days.
In fact, there is no small number of days in which an observer on Yavin IV's
surface will be able to look out at dawn and see themselves lined up with the dawn's edge
on the gas giant, and x number of days later they will see themselves in precisely that
position again. Thus the 49-day "month" has to be a sidereally-observed, not a solar
phenomenon. Logically, the solar days will not line up again until the whole system returns
to its precise sidereal starting point, which as we saw above,
is once every 49 Yavin years, or 637 standard ones.

**Sadovian Astronomy**

The system of astronomical observations and calendars developed by the Sith Lord Naga Sadow, during his exile on Yavin IV. (2)

When he wasn't whiling away his days genetically engineering nasty Sith monsters, Naga Sadow might have occupied himself with looking at Yavin IV's busy sky. He would have made the observations we have described above, and would quickly have determined that a sidereal, not solar calendar, would be the most effective way to go. Much of the Massassi temple complex could have been designed as a great sidereal observatory, set up to mark the positions of certain stars at certain points in the Yavin/YavinIV system's movements. Many other sidereal observations could be made using an accurate clock, and the planet and moon themselves:

- Time when a certain star is precisely on the horizon at sunup or sundown. (Opposite horizon from the sun, natch)
- Time when a certain star is precisely on the horizon at midnight.
- Time when a certain star is precisely at the zenith at sunup, sundown, or midnight.
- Time when Yavin IV is precisely lined up with the gas giant's dark/light edge (thus the planet appears to be precisely "half-full") and a certain star is precisely in line with the dark/light edge of Yavin, or just barely clear of Yavin's darkside edge at Yavin's equator. In order for the observer to be lined up precisely on Yavin's dark/light edge, this observation can be made only at Yavin IV's dawn or dusk.
- Time when the edge of the fully dark Yavin occludes a certain star.

All of these observations made together would have served well to enable the Massassi to keep track of Yavin's and Yavin IV's movements through the skies.

2)

Sidereal Vs Solar Time: U of Tennessee Astronomy 161

Length of a (Terran) Sidereal Year-- "Eric's Treasure Troves of Science"

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