◐ Does today's sky look like the sky I was born under?

Twelve signs for eight billion people is a fair complaint. Umbriel's free Birth-Sky Echo measures something more particular: how much today's sky resembles your own birth sky, planet by planet, placed among other dates so the number means something, with every error bar shown.

• What you'll needYour birth date. A birth time makes it sharper; without one it says exactly what that costs
• What you getA quiet daily meter of how closely today's sky resembles your birth sky, its place among random dates from 1900 to 2100 with a range, which planets line up with which, and now and then a flagged echo: when the sky last looked this much like yours, and when it next will
• CostFree, no account required

It is weather, not a target. Each day the sky is compared with the sky you were born under, body by body, and most days the honest answer is "about usual".

The definition is on the page, and so are its choices. Ten bodies, each counted by how near it is to its own birth place (exact is 1, 60 degrees or more away is 0), averaged with weights. The Sun counts three because it is the calendar; the outer planets barely move, so they count a quarter each. The weights are a choice and the page says so.

"High" only means something against other dates, so your birth sky is compared with a date every 4 days across two centuries, and today is placed among them: "more like your birth sky than 84 in 100 random dates", with the range it could honestly be.

An echo is a run of days in the top 1% of those dates. When one is near, the page says when the sky last looked this much like your birth sky, and when it next will. Tap either date to see the line-up for that day.

Every error bar is said. A day is not a moment, and the Moon moves about 13 degrees in one, so the meter carries a best and a worst case. A chart with no birth time cannot use the Moon precisely and cannot use any angle, and the page says so. Mercury, Venus, Mars and Pluto use lighter formulas that have not been checked against JPL, and it says that too.

It measures how alike two skies are and nothing else. It does not say what a sky like yours does, and it is not a test of you or of astrology.

Nothing is looked up. For each of ten bodies (the Sun, the Moon, and Mercury to Pluto) the app takes the angle between where the body is on the date and where it was at your birth, along the zodiac, the short way round. A body counts 1 if it is exactly where it was at your birth, 0 if it is 60 degrees or more away, and a straight line between. The ten are averaged with weights chosen and stated: the Sun 3, because it is the calendar; the Moon, Mercury, Venus, Mars, Jupiter and Saturn 1 each; Uranus, Neptune and Pluto a quarter each, because they stay within 60 degrees of their birth place for decades and a match says almost nothing about which day it is. The weights are a choice, not a law of nature, and the page says so. No angles and no houses are used: they turn once a day, so a date has no single one. To make a number mean something it is placed among other dates: your birth sky is compared with one date every 4 days from 1 January 1900 to 31 December 2100, and "more like your birth sky than 84 in 100 random dates" means 84 of every 100 of those dates resemble it less. An echo is a run of days in the top 1% of those dates, with runs less than 20 days apart joined, and the date given is the run's best day. The date is the sky at 12:00 UT on that calendar day, so everyone gets the same sky for the same date; the Moon moves about 13 degrees a day and is therefore blurred by about 6.6 degrees either way, up to 7.7. Your birth time is only as good as its source: a record is good to 5 minutes, family memory 30, a rough time 120, and an unknown time or zone is treated as 12 hours, which blurs the natal Moon by the same 6.6 degrees. Those blurs give a best case and a worst case for the day, both placed on the baseline, with the 95% Wilson range of the baseline's own sampling on the outside, and since baseline dates are not independent of each other that range is a floor. The Sun, Moon, Jupiter, Saturn, Uranus and Neptune are checked against JPL Horizons; Mercury, Venus, Mars and Pluto use lighter formulas that have not been measured against it, and the page says so. The search for echoes runs in your browser, in short slices, and the answer is kept on your device.