JEDDAH: We’ve all heard the expression “once in a blue moon” to denote something rare — and Saturday night was one of those special occasions, when the skies above the Kingdom were lit up with the spectacular full moon. What makes it a blue moon is not its color, however, but the fact that it is the second full moon to appear in this calendar month, a phenomenon that occurs every 2.5-three years. The term is also used for the third of four full moons in the season a blue moon, as each season contains three months. With Saudi Arabian skies mostly clear and the full moon rising around sunset on a weekend, residents were able to take the chance to venture out in the desert and stargaze as evening temperatures drop. Sky observers might also have noticed a reddish star nearby: Mars is vibrant and clear at present, and it is a good time to get a glimpse at the planet. The moon does really turn a blue color on very rare occasions, when “Earth’s atmosphere contains dust or smoke particles of a certain size, slightly wider than 900 nanometers,” according to EarthSky. This happened when the volcano Krakatoa erupted in Indonesia in 1883, and the particles of ash in the air gave the moon a blue hue.  People watched blue moons and purple sunsets for years, according to NASA.