Last fall, the ‘summer Milky Way,’ the brightest portion of the Milky Way that includes the center of our galaxy, appeared to slowly disappear into the sunset. However, its disappearance had nothing to do with its own movement or that of the Sun’s. The summer Milky Way’s disappearance into the solar glare was due to the movement of the Earth, as it orbits the Sun.
Copernicus figured this out in the mid-16th century when he proposed that the Earth orbited around the Sun instead of vice versa and that the stars were in the background, far beyond the Sun and the Earth.
You may be wondering how the Milky Way could have disappeared when it’s clearly visible in the southeast in February, as soon as darkness takes hold. That quandary is due to the fact that we are viewing the Milky Way from inside it.
The Milky Way is a spiral galaxy – a relatively flat disk consisting of a core and several arms that spiral around it. There are between 100 billion and 400 billion stars in the Milky Way, the Sun being one of them.
Our solar system sits in one of the Milky Way’s outer arms. This arm is known as the Orion arm. The Milky Way we see on February evenings, stretching from the southeast horizon, passing overhead, then dropping down to the northwest horizon, is a combination of the Orion and Perseus arms of the galaxy, sometimes called the ‘winter Milky Way.’ At this time of year, we’re looking away from the center of the galaxy.
Before dawn today, we had clear, dark skies – the first clear morning in some time. As I set up my camera and my eyes became dark-adapted, I was heartened to see the summer Milky Way rising on its side, just above the horizon. The Earth had moved far enough along its orbit around the Sun to bring the summer Milky Way back into the night sky, albeit at 4:30am MST!
– Eric Saltmarsh