Light inside: when the colours look like the photo
Short answer: Coloured light on the floor and columns needs direct sun shining squarely into the windows from fairly low in the sky. That happens at a different hour for each side of a building and shifts through the year, so we calculate it for each date and suggest the ticket slot.
Sagrada Família
Today: Red, orange and yellow light reaches deep inside from 12:50 to 17:05, strongest around 16:25
Pantheon
Today: At its lowest (11:55) the sun disc is on the dome above the entrance (north) side, about 31 m up
Sainte-Chapelle
Today: Deep blue and red light reaches deep inside from 10:35 to 15:25, strongest around 15:05
Notre-Dame
Today: Blue, red and violet light reaches deep inside from 10:15 to 16:20, strongest around 14:40
Cologne Cathedral
Today: The coloured-square pattern reaches deep inside from 10:00 to 15:20, strongest around 11:55
Nasir al-Mulk Mosque
Today: Pink, red, blue and gold light reaches deep inside from 08:00 to 11:45, strongest around 08:00
Light events on one or two days a year
Abu Simbel Sun Festival
Sunrise lights the sanctuary statues on 22 February and 22 October.
Pantheon on 21 April
The midday beam falls on the entrance doorway.
Chichen Itza equinox serpent
A serpent of light down the pyramid stairs at the equinoxes.
Newgrange winter solstice
Sunrise runs down the 19 m passage for a few mornings.
Stonehenge solstice sunrise
The sun rises close to the Heel Stone.
Manhattanhenge
The sun sets down the street grid.
Antelope Canyon light beams
Midday beams reach the canyon floor from spring to early autumn.
How we calculate it
You may read that light only reaches the middle of a church when the sun is between about 18° and 32° high. The idea behind it is right: the lower the sun, the further its light travels across the floor. The exact band depends on how high the windows are and which way they face, so we work it out for each building instead of using one rule.
For each building we measured which way the window walls face from OpenStreetMap outlines. For every 5 minutes of opening hours we compute how squarely the sun hits the glass and how far its light is thrown across the floor. For the Pantheon we trace the beam from the oculus onto the dome, the walls or the floor.