We just had our zero shadow day on the 19th November in Townsville. At solar noon, the suns zenith, vertical objects such as poles cast no shadow at all. It's a hot weather season too with 32-35C humid days and 24-28C very humid nights (Known locally as the "build up" or "silly season") (We are a coastal city, it gets more extreme inland in 'the outback')
Zero shadow days only occurs in the tropics, between the tropic of Capricorn (Lat 23.5o S) and Tropic of Cancer (Lat 23.5o N). In Hawaii it's known as "Lahaina noon" meaning "cruel sun".
It occurs twice each summer for my latitude of 19o15' South. The first is when the sun is moving south (Indian - "Dakshinayan"), the sun's travel stops at the tropic of Capricorn at our summer solstice around 22nd December (Near Rockhampton, Central Queensland) then it begins to move north again (Indian - "Uttarayan"). Our 2nd zero shadow day of this summer is on 22 Jan 2026 at 11:59am. After that date the sun's declination is north of our city, it reaches the equator (and equinox) on March 20, 2026.
It's probably no coincidence that most of Northern Australia's tropical cylones occur in January, February and March while the hot overhead sun (and longer days) heats up the Southern hemispheres oceans each year, (in our case it's the Coral sea's temperature to 30C or more) which equates to plenty of stored energy to power cyclones (also known as Hurricanes or Typhoons in other parts of the world).
I found a great app for my android phone "Zero Shadow Day finder" (ZSD icon). It was created by Alok Mandavgane, for the public outreach and education committee of the Astronomical Society of India.
It gives the two zero shadow days of each tropical summer (wet season) for a given location within the tropics. There are also other neat pages in the app, including an Introduction description page, a map animation and a 3D sun visualizer with a time-travel function.
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