Methodology and Accuracy

Where differences can come from

• Rounding: official tables round to the minute, and so do we on screen. A time that falls at 30 seconds past the minute can show one minute apart.
• Horizon and height: rise and set times assume a flat horizon at sea level. Mountains or tall buildings make the sun or moon appear later and disappear earlier; a high viewpoint or a sea horizon gives you a minute or two more.
• The air: refraction near the horizon depends on temperature and pressure; unusual conditions can shift a rise or set time by about a minute.
• City center: times are for the coordinates of the city center. Across a large city, sunrise can differ by a minute or more.
• Eclipse visibility: we can say whether an eclipse happens above your horizon, not whether the sky will be clear.

Our definitions

• Phase names: the four main phases are single moments; the whole local calendar day of that moment carries the phase's name. On other days the name follows the moon's angle from the sun.
• Supermoon: a full moon closer than 362,500 km. There is no official definition, so other sites draw the line slightly differently.
• "Dark at": the end of civil twilight, when the sun is 6° below the horizon.
• Golden hour: while the sun is between the horizon and 6° above it.
• New crescent: likely visible when, at sunset, the moon is at least 5° above the horizon and 8° from the sun. This is an astronomical estimate, not a sighting.
• Full moon names: North American names, with the Harvest Moon nearest the September equinox and a Blue Moon as the second full moon in a calendar month.

Traditional calendars

The Hindu tithi and the Islamic Hijri date are shown as astronomical information. Tithi times are the moments the moon reaches each 12° step from the sun; which day a festival or fast falls on follows panchang rules. The Hijri date uses the Umm al-Qura calendar of Saudi Arabia; many communities start months by local moon sighting, which can be a day later. For religious dates, we always point you to your local authority.

Moon images

The moon pictures are NASA's own renderings of the moon in each phase,^[50] which we turn to match how the moon looks from the place you choose. Eclipse photos are NASA images, credited where they appear.