Today is the summer solstice (or the winter solstice if you are in the northern hemisphere). To an astronomer, it’s the day when the sun is at its highest elevation relative to the celestial equator – but most of us just think of it as the longest day of the year.
During our visit to Paris in October, we went to the church of St Sulpice, where you can see the gnomon below. It is like a very specialised sundial which shows the elevation of the sun at about noon each day. If you were there today, you would see a spot of sunlight cross the meridian line (marked on the floor and obelisk) – at the end near the top of the obelisk.