Method
Where every number comes from, and how wrong it can be.
Solar longitude
The NOAA low-precision formula, accurate to about ±0.01°. Sign boundaries follow from it directly: the sun's position at noon UTC, divided by thirty degrees. This is why the sign dates published here change from year to year — because they do.
Lunar longitude
A truncated ELP2000 series (the principal periodic terms), carrying about ±0.2°. Enough to place a sign, not enough to be sure within roughly half an hour of a cusp — and where that happens the page says so rather than guessing.
The ascendant
Derived from Greenwich mean sidereal time, the coordinates of the birthplace and the obliquity of the ecliptic. The ascendant moves about a degree every four minutes, so both a time and a place are required; the offset in force at that place on that date — historical daylight saving included — is read from the IANA time zone database, and the resolved UTC instant is shown so you can check it.
What is not computed
No other planets, no houses, no aspects — so none are written about. A full natal chart needs ten bodies and a house system, which is outside what this site computes. Rather than ship a version that looks right and is not, it is not shipped.
Check it yourself
Every degree on every page is set in Plex Mono and can be compared against any ephemeris. If something does not match, that is this site's error — please say so.