I learned something interesting about moonrise last night. When I went to take a photo of the Pleiades star cluster, now coming up in the eastern sky around 8:30 p.m. local time, I also captured the glow of the moon, at the time about 2° (four moon diameters) below the horizon. The eastern sky glowed faintly, anticipating its rising.
Although that swath of sky appeared colorless to my eye, I looked at the camera’s playback screen and was surprised to see that it was actually colored rich orange-red. Then it dawned on me, if you excuse the pun, that moonlight is reflected sunlight, making moonrise a much fainter version of sunrise. Exactly the same way the pre-risen sun lights up the eastern sky in oranges and reds, a bright moon must necessarily do the same. To our eyes, that sky appears colorless only because there’s not enough light to stimulate the cones in our eyes that let us see the world in color.
In hindsight this shouldn’t have been a surprise, but since I rarely photograph the eastern sky before the moon comes up, I wasn’t prepared for this little revelation.
It’s so good to see the Pleiades return. This most distinctive star cluster becomes part of the evening skyscape starting every late September. There’s no naked eye sight like it: a tight bundle of stars that looks like dewdrops on a spider web. The sky abounds in star clusters including the naked-eye-bright Hyades, the Beehive and even the Southern Pleiades or IC 2602 in the constellation Carina. But the Pleiades’ compactness, brightness and “little dipper” form set it apart from everything else.
Also known as the Seven Sisters, the group is a physically related group of stars called an open cluster 444 light years from Earth. The light from the cluster you’ll see tonight left about the year 1573 right around the time of the famous supernova in 1572 observed and described by Danish astronomer Tycho Brahe. The supernova, which became as bright as Venus, appeared in early November that year and remained visible with the naked eye into 1574.
The Pleiades name most likely comes from the verb plein, meaning ‘to sail.’ The sailing season in the Mediterranean in ancient times began with the cluster’s rising in the east shortly before sunrise. It also has a Greek mythology connection — the seven bright stars represent seven sisters who were the daughters of the nymph, Pleione. As with so much in the sky, there’s a layer of myth beneath the frosting of science. Over 400 mostly faint stars are members of the cluster, but the brightest, the ones that seem to twinkle on cold nights, are hot, blue stars 40 to 1,000 times brighter than the sun. The cluster’s age is about 100 million years, making its first appearance in Earth’s sky during the Cretaceous period, when dinosaurs thumped about.
The cluster first comes up around 8:30 this week but will be up as soon as 7 p.m. by month’s end. Wait till around 9 or 9:30 p.m. and face northeast on the next clear night to see this clutch of sapphire stars. And don’t forget binoculars for an even better view. After the evolution and extinction of hundreds of species, countless natural disasters, wars and times of peace, the Pleiades are still around. Rising reliably and without a care on nights when water silently turns to ice.