Unlike our world, the universe of Celestial Matters obeys Ptolemaic astronomy, Aristotelian physics and Chinese Taoist alchemy. The earth is the center of the universe, with all the planets as well as the sun and moon revolving around it, set in a fixed sphere of stars. The timeline of this world also diverged from ours; the empire of Alexander the Great never collapsed, but grew and encompassed half the world.
A war has been raging between the Delian League and the Middle Kingdom for over a thousand years, both sides possessing different technology, but evenly matched with neither side being able to best the other. The protagonist, a Delian scientist named Aias, is heading Project Sunthief, an ambitious project that might finally bring about an end to the war. If all goes according to plan, the Delian League will fly a ship made from moonrock into space, take a piece of the sun and then use it as a weapon of mass destruction against the Middle Kingdom, obliterating their capital city and winning the war.
This novel provides examples of:
- Alchemy Is Magic: Aristotelian and Chinese Taoist alchemy as well. Taoist alchemy is used by the Middle Kingdom, but the Delian League base their technology off Aristotelian physics and don't really understand how the Middle Kingdom technology works.
- Alternate History: Alexander the Great ended up conquering pretty much all of Europe and then went into a thousand year war with the Orient.
- Anachronism Stew: The Delian League is a fundamentally Greek society, leading to joint rule by "Archons" as well as Athens and Sparta. The old Greek gods are still worshipped, but most of the League's R&D is focused in scientific research. Spartan soldiers still wear hoplite armor and carry swords, but use "evac throwers", which seem to be the League's version of firearms. Communication is still at the level of paper and pen, yet certain forms of aircraft are in common use and what seems to be analogous to a public transit system connects most of the Mediterranean. Perhaps most glaring of all, the League's space program is more advanced than real-life ones despite the fact that everything retains an old Greek aesthetic. This leads to scenes such as characters offering libations of wine to statues of gods onboard Chandra's Tear. Justified in that the League considers scientific advancement more important than that of society, as well as a number of actual modern technologies probably not being feasible under the setting's laws of physics.
- Applied Phlebotinum: A number of different materials are used in conjunction with the setting's physical laws for various applications; air-silver (explained as being silver mixed with particles of elemental air) is used for weather control, while celestial materials (coming from the planets) are used in the construction of spaceships. Fire-gold, made in a similar manner as air-silver, save with elemental fire, is used for clarifying the air or reducing air resistance, and is therefore incorporated into accurate artillery and "impellers", best described as this world's version of rocket boosters.
- Batman Can Breathe in Space: Justified. Space is actually filled with air, which steadily gets clearer and purer outward from the earth, in accordance with this universe's fictional physics.
- Black Vikings: Yellow Hare, Aias' bodyguard onboard Chandra's Tear, is a Cherokee-turned-Spartan soldier. Justified, however; a great deal of North America, including Cherokee lands, are League territory, and it is shown many times that the League values a person's ability to perform and aid in the war effort much more than their ethnicity.
- Cool Starship: Chandra's Tear, the ship built for Project Sunthief. Described as a mile-long teardrop, it is carved out of a single piece of moonrock and is the fastest spaceship (or rather, "celestial ship") ever built by the Delian League.
- Fantastic Nuke: The whole purpose of Project Sunthief. The piece of the sun that the Delian League plans to use against the Middle Kingdom is, in essence, a fusion bomb.
- Forever War: The war between the Delian League and the Middle Kingdom has been going on for a thousand years.
- Just Following Orders: There is a pivotal moment when a character has to decide whether to follow his orders or to obey his conscience.
- Oh, My Gods!: Since Christianity never seems to have come about in the book's setting, the characters tend to swear on the Greek gods.
- Only One Name: Most of the characters, keeping with old Greek tradition. Averted by Miiama Shizumi and Phan Xu-Tzu.
- Sufficiently Advanced Aliens: Zig-zagged in a very strange way: the reason the war between the Delian League and the Middle Kingdom has lasted so long is partly due to an inability for either empire to gain a significant edge over the other by way of reverse-engineering each other's technology, due to each operating on different principles. Therefore, each empire's science is only incomprehensible because of the aforementioned different operating principles, not due to the actual sophistication of the technology in question. This is mentioned several times as a major point of frustration for League scientists: Aias, who in our world would be both a physicist and astronomer, acknowledges the effectiveness of Middler tech, but is unable to actually grasp the mechanics that allows it to operate. Subverted later on when Aias and a Middler scientist come to terms and construct a hybrid celestial ship based on both sciences.