How we calculate

Positions, rising and setting

We calculate the Sun, Moon and planets with astronomy-engine, an open-source library that uses the VSOP87 planetary theory and a lunar theory derived from E. W. Brown's work; its authors test it against NASA JPL's Horizons ephemerides to within about one arcminute. Rise and set times use the top edge of the body and standard atmospheric refraction at sea level, as the US Naval Observatory does; in our checks for Denver, Miami, New York and Fairbanks on several dates, sunrise, sunset, moonrise, moonset and twilight agreed with the Naval Observatory to within a minute. Elevation and local terrain can shift real rise and set times by a few minutes.

Darkness and twilight

"Full darkness" is astronomical night, when the Sun is at least 18° below the horizon. Civil twilight ends at 6° and nautical twilight at 12°. North of about 49° latitude in early summer, and across much of Alaska for months, the Sun never gets 18° below the horizon, so there is no full darkness; pages say so. "Moon-free darkness" is the part of astronomical night with the Moon below the horizon. The best stargazing nights in each month are those when the Moon is down for at least 60% of full darkness.

Planets

A planet counts as visible when it is at least 10° above the horizon after civil dusk or before civil dawn (Uranus and Neptune: after astronomical dusk, and they need binoculars or a telescope). We sample the night every 10 minutes and report when it is up, when it is highest, in which direction, its brightness (magnitude) and its constellation.

Meteor showers

Shower dates, peak rates (ZHR) and radiant positions come from the International Meteor Organization's 2026 and 2027 calendars and working list. Where the calendar gives a peak time we use it; otherwise we calculate the moment the Sun reaches the peak's solar longitude (J2000), which reproduces the IMO's published peak times. For each city we find the night nearest the peak, then the hours when the radiant is within 15° of its highest point in a dark sky (Sun at least 12° down), avoiding moonlit hours when the Moon is more than 30% lit. "Up to N per hour" is the ZHR scaled by the sine of the radiant's altitude: what an observer under a very dark sky could see. Light pollution, haze and a limited view usually cut that to a half or a third, and a bright Moon cuts it further, which our outlook rating takes into account.

Northern lights

NOAA's space weather scale says how far from the magnetic pole aurora is usually seen at each storm level: about 60° geomagnetic latitude at G1 (Kp 5), 55° at G2 (Kp 6), 50° at G3 (Kp 7), 45° at G4 (Kp 8) and 40° at G5 (Kp 9). We compute each city's geomagnetic latitude from the dipole pole position of the IGRF-14 model for 2025 (80.85° N, 72.62° W) and interpolate the Kp needed for aurora low on the northern horizon. Poleward of 60°, where the auroral oval itself passes overhead on quiet nights, the level falls faster, to Kp 1 at 66°. Tonight's chance compares the highest Kp in NOAA's 3-day forecast during the night's darkness with that level: "possible" when the forecast reaches it, "camera only" within one level below, because phone cameras record aurora too faint to see. Forecasts more than a day ahead are uncertain.

Clouds

Cloud cover is the National Weather Service's gridded sky-cover forecast for the exact location, averaged over the recommended viewing window. It is available for the US only and is fetched by your browser when you open a page.

ISS passes

We download the space station's orbital elements from CelesTrak and propagate them with the SGP4 model. A pass is listed when the station is at least 10° high and sunlit while the Sun is at least 6° below your horizon; brightness is estimated from its distance. Times agree with other pass predictors to within about a minute, but the station's orbit is adjusted from time to time, so passes more than a few days ahead can shift.

Eclipses and Moon phases

Eclipse circumstances for each city (start, greatest and end times, how much of the Sun is covered, and whether the Moon is up for a lunar eclipse) come from astronomy-engine's eclipse searches; the list agrees with NASA's eclipse predictions by Fred Espenak. Moon phases are exact instants. Full moon names follow the traditional US list; the Harvest Moon is the full Moon nearest the September equinox and the Hunter's Moon the next one. A supermoon, on our pages, is a full Moon within 10% of the year's closest approach measured across that year's range of distances; other publishers use other rules.

The score for tonight

The score adds up to 10 points:

• Darkness (up to 4 points): hours of full darkness with the Moon down count fully; dark hours with the Moon up count in proportion to how little of the Moon is lit.
• Events (up to 5 points): a meteor shower peak (up to 3, by the outlook rating), northern lights possible (3) or camera-only (1.5), each bright planet up (0.5, up to 1.5), a partial or total lunar eclipse (3), a bright ISS pass (0.5).
• One point to start, because any clear night has something to see.

Then clouds: the total is reduced by 0.9% for every 1% of forecast sky cover, and a sky more than 85% covered scores at most 2. Without a cloud forecast, the score assumes clear skies and says so. The score is a guide to what the sky offers, not a guarantee: a clear moonlit night is still good for the Moon and bright planets.

Cities and locations

We cover 464 cities from the US Census Bureau's 2025 place data: every city of 50,000 people or more that is not within 30 km of a larger listed city, towns of 20,000 or more that are more than 80 km from any listed city, and state capitals. Any other US place works through its ZIP code (Census ZIP code areas) or your device's location, with the same calculations.