The Qixi Festival, also known as Chinese Valentine's Day, is a day to celebrate the annual meeting of a kind young cowherd named Niu Lang and a fairy named Zhi Nyu, a girl whose weaving skills are incomparable.

It falls on the 7th day of the seventh month on lunar Chinese calendar, which is Tuesday this year.

According to the legend, the young couple, after being separated by the godly empress, would meet each other on this day every year with the help of thousands of magpies who, touched by their love story, create a bridge for them to walk across.

Read the story of Niu Lang and Zhi Nyu.

That's where the name of the relay satellite for China's Chang'e-4 lunar probe "Queqiao," meaning Magpie Bridge, comes from.

However, romance is romance.

Astronomically, Niu Lang, represented by Altair, is the head of the eagle constellation Aquila, while Zhi Nyu, symbolizing the star Vega, belongs to the constellation Lyra.

The two stars are about 16 light years apart. That means it takes 16 years even for the light to travel from Altair to Vega.

So far, the world's fastest spacecraft is the U.S.'s Parker Solar Probe, which NASA said can hurtle around the Sun "at approximately 700,000 kph" at closest approach. And such a speed is only 0.064 percent of the speed of light.

So actually, it's impossible for Niu Lang and Zhi Nyu to meet once a year with our current technology.

But we on Earth can see the two brilliant stars when weather permits. They are both the brightest in their own constellations.

The Milky Way passes between them, flanked by Vega on the western side and Altair on the eastern side.

Together with a third star called Deneb, the trio forms an asterism dubbed the Summer Triangle, which is noticeable in the evening sky beginning around June.

Magpie Bridge

The folktale also said the magpies disappear on the day of Qixi because they go to build a bridge for Niu Lang and Zhi Nyu.

It's true that we see fewer magpies on that day, but not because they go to space. 

Birds molt, and for magpies the process usually starts in early July of the lunar calendar, which is around the Qixi Festival.

Lots of feathers fall off and are replaced with soft and fine ones, to resist the cold winter.

Magpies are vulnerable during molting and their mobility is also greatly reduced. Therefore, they stay in their nests.

But still, if magpies really could help, then how many magpies would it take to build a bridge across the Milky Way to connect the couple?

The distance between Altair and Vega is about 16 light years away, and magpies' average body length is 45 centimeters. So if lined up, roughly a total of 34,000 trillion magpies would be needed.