DuoCards vs Zero Shadow Day

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Between the Tropic of Cancer and the Tropic of Capricorn, the Sun passes exactly overhead twice a year. On those two days, at local noon, a vertical pole casts no shadow at all. That is a Zero Shadow Day. Zero Shadow Day tells you when yours is, and shows you why it happens. ZSD FINDER Your two zero shadow dates, with the exact local time of each, and how far you are from the equator and from the tropic. Tap anywhere on the world map to ask the same question of any other place. LOCAL NOON The moment the Sun is highest where you are — rarely 12:00, and the reason why is worth knowing. SHADOW LENGTHS The shadow of a one-meter pole through the whole day, drawn as a curve. Drag the time, drag the latitude, and watch the curve answer. SHADOW FOR YEAR The same shadow across all 365 days, so the two zero-crossings appear where the curve touches zero. ANIMATIONS Watch the zero shadow line sweep north and south across the Earth through the year, and see the equation of time and the analemma — the figure of eight the Sun traces in the sky. VISUALISATIONS The celestial sphere in three dimensions: the Sun's daily arc, the celestial equator and the ecliptic, from your own latitude. ZERO SHADOW MOMENT A globe showing where on Earth the Sun is directly overhead right now. Run it forward and follow the point around the world. Every calculation runs on your device. There are no accounts, no advertising and no tracking of any kind. Sharing your location is optional — pick a city from the list instead and everything works the same. Available in English, Hindi, Marathi, Tamil, Telugu, Kannada, Gujarati, Spanish and Portuguese, with a language switch inside the app. Made by Alok Mandavgane for the Public Outreach and Education Committee of the Astronomical Society of India, in support of its Zero Shadow Day campaign.