Our morning twilight all-sky chart for September, viewable below, shows the changes in positions of the naked-eye planets and the stars of first-magnitude or brighter, less than an hour before sunrise.
Changes are caused by the motion of the Earth and the other planets in their orbits. Stars will appear to drift from east to west across the sky as weeks pass, thanks to the revolution of Earth around the sun. Venus now dominates the predawn sky, but is getting a little lower each morning, because it is heading toward the far side of the sun, where it will arrive in January 2018.
Annually in September and October, the huge Winter Hexagon—in clockwise order from its brightest member, Sirius, Procyon, Pollux and Castor, Capella, Aldebaran and Rigel, with Betelgeuse inside—is well-placed high in the sky before dawn. The flashing blue-white Dog Star Sirius is its most prominent and southernmost member. Sirius is the brightest star in Earth’s night skies, but it does not appear as bright as Venus.
Morning planet gatherings: Low in the eastern morning sky in early September, Mars, Mercury and Regulus emerge to the lower left of Venus. Regulus, at magnitude +1.4, is the faintest of the first-magnitude stars. Mars is now even fainter, at magnitude +1.8, as dim as it gets—but Earth will catch up to it and close the gap. In July 2018, Mars will gleam at magnitude -2.8, visible all night, in the southwest at dawn (and southeast at dusk), and will be closer to Earth and brighter than at any time since the very close approach of August 2003.
On Sept. 4, Mercury, at magnitude +1.7, is a close match in brightness to faint Mars; they rise nearly at the same time, with Mars 3 degrees to the left of Mercury, and Regulus below them. Binoculars will show them in twilight about 17 degrees to the lower left of Venus, with the contrasting colors of reddish Mars and blue-white Regulus. All rise earlier each morning and get easier to see, especially Mercury, which brightens…