Inner Sphere world with extreme tidal range

Image from Steve Bowers
Midgard and two of its moons Valhalla (centre) and Asgard

Name: Alpha Caeli A
Distance from Sol: 65 ly
Type: F2V
Luminosity: 5.84 x Sol
Companion: Alpha Caeli B
Type: M0.5
Separation Between A and B: 280 AU

Name: Midgard
Semimajor Axis: 2.479 AU
Orbital Period: 3.027 standard years
Diameter: 10058 km
Surface Gravity: 0.76 gees
Mass: 0.467 x Earth
Rotation Period 17hrs 58 minutes
Moons: Asgard, diameter 451 km; Valhalla, diameter 1012 km, Loki 2342km

Midgard is an Inner Sphere world orbiting Alpha Caeli some 65 light years from Sol.
It was terraformed in the early Middle Federation period by descendants of Northern European refugees from the Great Expulsion.


Midgard is orbited by three moons, Asgard, Valhalla and Loki. All three have been terraformed using the worldhouse enclosure method.

Asgard, the innermost moon, orbits Midgard in a near-synchronous orbit with a semi-major axis of 27,437.2680 kilometers, giving it a period of 18 hours, 23 minutes, 4.3607 seconds. The moon has a radius of 225.5 kilometers, a mass of 1.5138e+20 kilograms, and a bulk density of 3151.6192 kilograms per cubic meter; by itself, it can raise an 8.8378-meter mid-ocean tide on Midgard.

Valhalla, the second moon, orbits Midgard in a circular orbit with a semi-major axis of 71,059.5356 kilometers, giving it a period of 76 hours, 36 minutes, and 10.2508 seconds. Larger and slightly more dense than its inner neighbor, Valhalla has a radius of 506.0 kilometers, a bulk density of 3362.5966 kilograms per cubic meter, and a mass of 1.8248e+21 kilograms; by itself, Valhalla would raise a mid-ocean wave on Midgard 15.8830 meters high.

Loki, the outermost satellite (semi-major axis = 231922.1841 km), is the largest (radius = 1171.2 km) and most massive (2.1997e+22 kg) in the system, and has the longest period (450h03m01.8331s). By itself, it would raise a mid-ocean wave on Midgard 17.9741 meters high.

Asgard and Valhalla come into conjunction every 76h36m10.2508 sec (one Valhalla orbit and approximately four and one-sixth Asgard orbits), resulting in a mid-ocean tidal height of 24.7208 meters. As Loki's orbital period is nearly 24.5 times that of Asgard and about 5.875 times that of Valhalla, there is no true mean-motion resonance, but the three moons can come into near-conjunction at times, giving a mid-ocean wave height of up to 42.6949 meters.

Image from Steve Bowers
Midgard - geography

The geography of Midgard often causes an amplification of these tides, especially in the long funnelling bays of the northern hemisphere. The maximum tides on this world only occur on the rare occasions when the triple conjunction occurs above Malmo Bay; tides can reach as high as 1900 metres on occasion, the highest tides in the Inner Sphere and much of the Middle Regions.

As a consequence Midgard is perhaps best known as the site of the All-Worlds Surfing Championships held when the tides are exceptionally high. The event attracts baseline and near-baseline athletes from all over the Inner and Middle Regions and is transmitted via the Known Net as far out as the Periphery.

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Development Notes
Text by Todd Drashneradditional material by Steve Bowers and Radtech497
Initially published on 07 January 2002.