Prague's solstice spectacle: Witness a centuries-old astronomical show at Charles Bridge

Prague has an astronomical show that has been running nonstop since the 14th century. The “Prague solstice mystery” occurs every year on the first day of astronomical summer, which this year is June 21. If you stand at the Old Town Tower of Charles Bridge and look toward Prague Castle, the sun sets over the roof of St. Vitus’ Cathedral, right over the main altar.

Emperor Charles IV was a big believer in astrology and superstition. When he planned his building projects, he often infused them with a mystical element. The location of Charles Bridge was shifted slightly from the previous Judith Bridge, in part to facilitate the alignment.

The solstice is the longest day of the year and also the furthest the sun sets in the northwest before heading back southwest. The sun sets in slightly different spots every day and will reach its furthest to the southwest on Dec. 21, the winter solstice. It then starts its slow march back to the Castle.

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 bell tower over the small altar; and disappears behind the roof before the edge of the sun peeks out of the church’s apse. It then sets behind the roof of the adjacent All Saints’ Church.

While the official sunset time is after 9 p.m., 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 p.m.

A decade or so ago, just one or two people would show up for the event but word has gotten out about it, and now there are usually crowds on the bridge and in Křižovnické náměstí, the square next to the bridge.

If the sky is completely overcast you can go a day or two earlier or later to see pretty much the same effect. This year, the forecast is for rain on June 21, but partly cloudy skies on June 20. A few clouds make for better pictures, as they can catch the colors of the setting sun.

You can also watch the event online in color since a live camera is installed on top of the Old Town bridge tower, streaming 24 hours a day. It flips direction but will be showing the Castle view half the time.

In English, the alignment is sometimes called the Prague Stonehenge, referring to the stone circle in Britain that also has astronomical alignments. In Czech, it is called “Pražské slunovratové mystérium” – the Prague solstice mystery. The word “mystery” in this case is used in the religious sense of a hidden spiritual aspect.

The alignment isn’t due to 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. Previously, it was called the Stone Bridge or Prague Bridge.

Astrologers allegedly picked the time that construction started on Charles Bridge 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.

Architect Petr Parléř worked on both St. Vitus’ Cathedral and Charles Bridge, so he would have been able to ensure they lined up. A statue of St. Vitus in a large circle representing the sun is on the facade of the bridge’s Old Town tower. This creates a further link between the sun, the tower, and St. Vitus’ Cathedral.