Sources, licences and accuracy

Yuniverse shows two kinds of thing, and keeps them apart. Sky is measured: positions, phases, times and directions, calculated from published astronomy. Story is traditional: zodiac signs, tarot, numbers and other symbolic systems, offered for reflection and fun. Story is never presented as science or prediction.

Where the measured numbers come from

Sun: Jean Meeus, Astronomical Algorithms (2nd edition, 1998), chapter 25. Good to about 0.01°.
Moon and its phase: Meeus chapters 47 and 48, with every term of the published tables. Good to about 10 arcseconds; checked against the book's worked example to 0.00001°.
Planets, Mercury to Pluto: E. M. Standish (NASA JPL), Keplerian Elements for Approximate Positions of the Major Planets. Good to a few arcminutes, and only valid from 1800 to 2050, which is why Yuniverse covers those years and no others.
Earth's orientation: Precession, nutation, aberration and sidereal time from Meeus chapters 12, 21, 22 and 23.
Earth's rotation (ΔT): Espenak and Meeus (2006), NASA eclipse polynomials.
Atmosphere and viewpoint: Positions near the horizon include atmospheric refraction (Sæmundsson's formula). The Moon's position includes parallax for an observer on Earth's surface, which moves it by up to 1°.
Rise and set times: When the top edge of the Sun or Moon meets a flat, clear horizon; planets at their centre. Real horizons, hills and weather change this by minutes. Checked against an independent algorithm to within about a minute.
Stars: The Yale Bright Star Catalogue, 5th revised edition (Hoffleit and Warren, 1991), all 5,080 stars down to magnitude 6.0, from the NASA Astronomical Data Center catalogue V/50. Public domain. Positions are moved from the year 2000 to the date shown; the stars' own slow motion (at most a few arcminutes over 250 years) is not applied.
Star names: The 40 names shown are IAU-approved, each checked against the catalogue's own designation.
Constellation figures: From d3-celestial by Olaf Frohn (licence below). Figures are a drawing convention, not boundaries.
Constellation boundaries: The official IAU boundaries (Delporte, 1930), searched with the method of N. G. Roman (1987, PASP 99, 695; CDS catalogue VI/42). This is how Yuniverse says which constellation the Sun or a planet is really in. Checked against Roman's worked examples and against all 3,143 catalogue stars whose names include their constellation: every one agrees.
Places: About 34,000 cities and towns of 15,000 people or more from GeoNames (CC BY 4.0), each with its own time zone, searched on your device.
Time zones: The IANA time zone database (version shown on the birth sky page), including every historical change of clocks and daylight saving since 1800 that it records. Before standard time, places kept local mean time, and so does the calculation. One reference city per time zone carries the coordinates; you can enter your own.

What we assume

The sky is drawn as seen from sea level on a clear night, with a flat horizon.
Brightness and colour of stars are shown as they would look to the eye, softened near the horizon; this is a picture, not a photometric model.
A birth time given as approximate stays approximate: the sky is drawn for the middle of the window, the trails show the rest, and any sign that could change inside the window is shown as a range.
A local time that happened twice (clocks going back) or never (clocks going forward) is asked about, not guessed.
Signs are the tropical zodiac: twelve 30° slices of the ecliptic measured from the March equinox. They no longer line up with the constellations of the same names.

Astrology conventions (the Story chart)

Astrology is a tradition, not a measurement. These are the conventions the Story chart uses, shown so you can see exactly what is a choice.

Zodiac: Tropical: twelve equal 30° signs counted from the March equinox, as in modern Western astrology. Signs are not constellations; both are shown, side by side.
Positions: The same calculated positions as the sky pages (apparent, geocentric ecliptic longitude). Nothing is recalculated for astrology.
House systems: Placidus (time-trisection of each degree's semi-arc; the most widely printed modern system, undefined beyond about 66.5° latitude) and Whole Sign (each house one whole sign from the Ascendant's sign; the oldest system in use). You choose; they are never mixed.
Aspects: Conjunction 0°, sextile 60°, square 90°, trine 120°, opposition 180°. Orbs: Tight (6/6/5/5/3°), Standard (8/8/7/7/5°, the default), Wide (10/10/8/8/6°), for conjunction, opposition, trine, square and sextile. Astrologers set orbs differently; no orb is scientifically determined.
Uncertain birth times: Everything that depends on the time is calculated at both ends of your window. If it differs, a range is shown. With no time, no Ascendant, Midheaven or houses are produced.
Interpretations: Short attributed descriptions of what Western astrology traditionally associates with each sign, planet, house and aspect, written by Yuniverse from widely shared traditional themes. They are not predictions or advice and do not describe you.

Dream journal

Symbol readings are attributed to two traditions: the popular dream-book tradition, and Jungian interpretation (C. G. Jung's school of analytical psychology). They are summaries written by Yuniverse, not quotations, and they are not explanations of your dreams. Patterns are counted only from what you recorded; nothing about your mood or health is inferred. The Moon of each dream night is calculated for 3 a.m. local time on the date you choose.

Type

Marcellus (Astigmatic) and Cormorant Garamond (Christian Thalmann), both under the SIL Open Font License, and Plus Jakarta Sans for the interface.

Privacy of these calculations

Every calculation on the sky and birth sky pages runs in your browser. Your birth details and your location are not sent to Yuniverse or anyone else. Location is only asked for when you press the button, and is rounded to about a kilometre. If you choose to keep a birth sky, it is stored on your device only, and you can forget it at any time.

Licences

The Yale Bright Star Catalogue is in the public domain. The IANA time zone database is in the public domain. Place data © GeoNames (geonames.org), licensed under CC BY 4.0; Yuniverse uses a reduced copy (names, regions, coordinates, time zones, population). Constellation boundaries: CDS catalogue VI/42 (Roman 1987). Constellation figures and names come from d3-celestial:

Copyright (c) 2015, Olaf Frohn
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Fonts: Marcellus, Cormorant Garamond and Plus Jakarta Sans, all under the SIL Open Font License. Brand art: Yuniverse.