Beltane kicks off the astronomical events for May. For millennia, the Celts recognized the four cross-quarter days – Imbolc, Beltane, Lughnasa, and Samhain – as important markers for the beginning of each season. Celebrated when the Sun reached the mid-way point between equinox and solstice, the date of this solar station may well have occurred closer to the first of each month than they do today. But since contemporary culture no longer relies on celestial mapping as our calenders, today's incarnations of these ancient customs (Groundhog Day, May Day, Lammas, and Halloween) are now fixed.
Because it's an event that focuses on the Sun's position, the lunar calender doesn't always coincide with these pagan festivals. This year, however, we'll have a full moon on May 3, just two days after the traditional May Day / Beltane celebration. Often called the "Flower Moon" or "Planting Moon," the second full moon following the spring equinox will light up the spring night from Libra. Saturn, lighting in Scorpius, will follow it closely, meeting up with the waning gibbous moon the next evening.
Since the beginning of 2015, our full moons have been occurring early in the month, a phenomenon that will continue through July, when two full moons will switch up this pattern. The second full moon – a "Blue Moon" – will occur on July 31.
The moon's waning gibbous phase will provide star-gazers a great opportunity to spot elusive Mercury which will reach its greatest distance from the Sun on May 6. This position is particularly good for sky-watchers in the Northern Hemisphere. However, while Mercury has outpaced the Sun, the planet still won't be easy to spot unless you have an unobstructed view of the western horizon after nightfall. However, if all conditions are perfect, try looking for Mercury, who will be hanging close to the horizon some 60 minutes after the Sun has set, lighting close to the Pleiades, the beautiful open star cluster located in Taurus.
Venus's radiant evening apparition continues this month as she moves away from the stars of Taurus into Gemini on May 8. By mid-month, she's lighting west for almost 2 1/2 hours after twilight fades, giving us a wonderful opportunity to appreciate one of our night sky's brightest luminaries – second only to the moon – as she races past Gemini's stellar brace, Castor and Pollux, and heads toward Jupiter.
Jupiter stands well past its zenith at sunset as May begins. Traveling all month with the faint stars of the constellation Cancer, Jupiter is dragging its heels, waiting for Venus to catch up with him for a beautiful conjunction at the end of June. But even by the end of this month, the two will set a dazzling display in the western sky that won't set until near midnight.
The waxing crescent moon makes its first appearance in the west on May 19, far below Venus, swinging closest to the lovely 'Evening Star' on May 21. Two days later, the moon will light next to Jupiter, moving over to stand by Regulus, the bright heart of constellation Leo, the next evening, May 23.
Not long after twilight fades from the west, Saturn rises east as May begins with the stars of Scorpius. Although it remains close to Scorpius, by mid-month, the ringed planet has twisted back into the constellation Libra, reaching its annual opposition to planet Earth on May 22. On the final day of May, as the full moon of June approaches, the waxing gibbous moon and Saturn travel companionably across the late spring's sky.