Prague uncovered: See the Prague solstice mystery from Charles Bridge

Emperor Charles IV was a big believer in astrology and superstition, and evidence of that can still be seen today. He was behind the summer solstice alignment of the Old Town Tower of Charles Bridge and the altar of St. Vitus’ Cathedral at Prague Castle.

This year, the summer solstice is June 21. While the official sunset time is after 9 pm, if you want to see the sun cross the tops of the cathedral towers and set on the roof, you need to get to Charles Bridge a little after 8 pm, as the Castle is on a hill. The whole show is over around 8:30 pm.

In English, the alignment is sometimes called the Prague Stonehenge, referring to the stone circle in Britain that also has astronomical alignments. In Czech, the even is called “Pražské slunovratové mystérium,” meaning the “Prague solstice mystery.”

If you stand at the Old Town side of Charles Bridge and look toward Prague Castle, the sun crosses the main tower of St. Vitus’ Cathedral, lighting up the top chamber, goes past the small altar bell tower, and sets on the roof before the edge of the sun peeks out of the church’s apse and then sets a second time on the roof of the adjacent All Saints’ Church.

New this year, you can watch the event online in color, as there is a live camera installed on top of the Old Town bridge tower streaming live video 24 hours a day. It periodically flips direction but will be showing the castle view half the time.

Clouds sometimes get in the way, so if the weather looks unfavorable you can go a day or two earlier or later to see pretty much the same effect. This year, the forecast is for partly sunny skies on June 21. A few clouds actually make for better pictures, as they can catch the colors of the setting sun.

The sun sets in a slightly different location every day. Starting with the winter solstice, around Dec. 21 every year, the sun sets a little bit more to the east every day, reaching its furthest point on the summer solstice, around June 21. Then it starts to head back west every day, until it reaches the spot where it started on Dec. 21.

So from the point of view of the Old Town side of Charles Bridge, the sun reaches the roof of St. Vitus’ Cathedral and goes no further.

The alignment seems to be intentional, rather than the result of random chance. Emperor Charles IV was instrumental in building both St. Vitus’ Cathedral and what is now called Charles Bridge, and by all accounts, he took astrology very seriously, as was typical for the era he lived in. (The name Charles Bridge wasn’t used until 1870; before that is was the Stone Bridge or Prague Bridge.)

Astrologers allegedly picked the time that construction on Charles Bridge began so it would create a numerical palindrome of odd digits: 1357-9-7-531, meaning the year 1357 at the 9th day of the 7th month at 5:31 in the morning in Old Bohemian time.

Charles Bridge took the place of the previous Judith Bridge, which was so badly damaged in 1342 in a flood that it could not be repaired. The route of the bridge changed, though. The Malá Stana side remained the same, and one of the towers on that side is from the Judith Bridge era.

On the Old Town side, the bridge was moved downstream slightly. This made the bridge stronger against the current, but it also facilitated the solstice alignment. The shift in the angle explains why the path from one bridge tower to the other isn’t straight.

Architect Petr Parléř worked on both the cathedral and the bridge, so he would have been in a position to make sure they lined up. One of the statues on the facade of the bridge’s Old Town Tower is of St. Vitus, and he is depicted in a large circle representing the sun. This creates a further link between the sun, the tower, and the cathedral, which is named for the same saint.

As with all events involving the sun, it is important not to stare directly at the sun for too long. Taking periodic glances while wearing dark sunglasses is recommended, and another option is to watch the sun through the screen of your phone or camera while avoiding directly looking at the sun, or watching it online.