It’s a large waxing gibbous moon that lights up the evening sky right now. The moon is moving toward full phase, which will arrive on July 22, 2013. This is the second full moon after the June solstice. There are usually three full moons in between a solstice and an equinox, or vice versa. Seven times in 19 years, however, four full moons fall in a single season. In that case, the third of a season’s four full moons is also called a Blue Moon. The next Blue Moon by this definition will happen on August 21, 2013.
A more recent definition of blue moon refers to the second of two full moons to occur in a single calendar month. By this definition, the next blue moon will be forthcoming on July 31, 2015.
At tonight’s phase of the moon, we in North America see roughly 90% of the lunar disk illuminated in sunshine tonight. Only about 10% of the moon appears engulfed in shadow. If you could see the far side of the moon, you’d see just the opposite: 90% of the lunar disk in shadow and 10% illuminated by sunshine.
The moon has a day side and a night side just as Earth does. We on Earth say that the moon waxes and wanes, but this is, in a way, a charming fiction. The reality is that the moon is always half illuminated, just as Earth is. From Earth, we see various fractions of the moon’s day side, as our vantage point of the day and night sides of the moon shift as the moon orbits around Earth.
Now maybe you can see why the far side of the moon is not the same thing as the dark side of the moon.
The near side of the moon – the side that we see from Earth – is sometimes illuminated, and sometimes dark. The far side of the moon – the part that we don’t see from Earth – is also sometimes illuminated, and sometimes dark. A full half of the moon is always illuminated, but this day side shifts around and around the entire globe of the moon – just as Earth’s day side shifts around and around our globe, endlessly. The far side of the moon is only the dark side of the moon at full moon.
The terminator – the shadow line on the moon that divides day from night – shows you where it’s sunrise on the moon when the moon is waxing from new moon to full moon.