On This Instrument
Ephemeris is a perpetual orrery — a working model of the solar system, engraved in light rather than brass. Set the date instrument to any day between 1800 and 2200 and the eight planets take up the stations they truly occupied, or truly will. Let it run, and they proceed at their proper relative gaits: Mercury completing his 88-day errand while Neptune, in no hurry whatever, spends 165 years on a single circuit.
A confession regarding scale
Space is chiefly composed of space. Were this plate honest about distance, with Neptune at its rim, Mercury would huddle scarcely a hair's breadth from the Sun and the four stony worlds would be lost in his glare. We have therefore taken the logarithm: each orbit is placed by the logarithm of its true mean distance, which preserves the order of the heavens while abandoning their emptiness. The planets' disks are likewise enlarged some thousands-fold, and the Moon's little orbit is drawn indulgently wide — else she would hide inside the very pixel that is the Earth. Time alone we have not flattered. Every period, every alignment, every return is kept true.
On the mathematics
Positions derive from mean orbital elements at epoch J2000. Each planet's mean longitude advances as L = L₀ + n(t − t₀), where n is its mean daily motion; the equation of centre, carried to third order in the eccentricity, then converts this uniform march into the true, slightly hastening-and-slackening motion along the ellipse. A given date therefore always shows the same configuration — scrub away and return, and you will find everyone precisely where you left them. Across these four centuries the longitudes hold to within about a degree, which is ample for an atlas and better than most brass.
On the foretold conjunctions
The almanac line at the foot of the instrument watches the simulation itself and names the next occasions on which two planets will share the same solar longitude — conjunctions as the Sun would see them, note, which differ somewhat from the meetings in our evening sky. The sign ☌ is the old notation for such an appointment.
On what is drawn
Every mark on this plate — the stippled stars, the zodiac dial, the hatched portraits, Saturn's rings — is drawn by the instrument itself at the moment you open it. No photograph, no engraving, no borrowed image appears anywhere; the atlas is computed fresh, as the heavens are.
Whitlock & VaneNo. 11 Lantern Court, London · est. MDCCXCIV
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