Like the motionless yet shifting grains of sand on a beach, the celestial sphere is where change and continuity coexist. Stargazing has long nourished the notion of eternity in our cosmologies, but it’s the motion of objects in the Solar System that defines our relationship to time.
The autumnal equinox on September 22 is the perfect example: At 3:21 p.m., the alignment of the Earth’s equator with the Sun, about 150 million kilometres (or one astronomical unit) further away, marks the start of a new season. But we can also deepen our understanding of our place in the cosmos by looking skyward, at the astronomical distances—corresponding to both space and time—that separate us from everything floating around out there.
Let’s dive right in
This September, the closest celestial object visible to us will (unsurprisingly) be our loyal sidekick, the Moon. For the lunar cycle beginning with the new Moon of September 6, the 11th is when our natural satellite’s elliptical orbit brings it closest to us, at 368,463 km. The Moon will be a waxing crescent, visible due west in the evening. While there’s something dramatically beautiful about a full Moon rising, its large apparent size at that moment is deceptive. If you have the chance to admire the majestic full Moon of September 20 rising from the horizon above distant buildings, don’t be fooled: It will in fact be about 20,000 km further away and have an apparent diameter 10% smaller than the crescent of September 11.
To catch a glimpse of celestial bodies located further away, we need to shift our focus to the planets. With Mars tucked behind the Sun and Venus slowly emerging from superior conjunction, elusive Mercury is our next target. Your best bet for observing it will be during the first few evenings of the month, in the minutes following sunset: Look for a small speck of light midway between the very brilliant Venus and the Sun below the horizon. On the evening of September 8, a very thin lunar crescent overhangs the half-illuminated tiny planet. Mercury continues drawing closer to Earth and comes to within 0.7 AU (astronomical units) by the end of the month, but its proximity to both the Sun and the horizon makes it unobservable after mid-September.
Next up is Venus, which is also steadily approaching Earth and will be 0.89 AU away from us by month’s end. The dazzling planet draws our attention at twilight, despite the shallow angle of the ecliptic to the west-southwestern horizon at this time of year. Venus first flirts with Spica, passing less than 2 degrees from the star from September 4 to 6, and then has a meet-up with the crescent Moon (only four degrees of separation between them) on September 9.
Despite Venus’s spectacular glow at twilight, Jupiter is the undisputed star of the late summer sky due to its more favourable position in the middle of the night. It’s also our next celestial target, about 4 AU away. Look for the giant planet as it rises above the east-southeastern horizon in the minutes after sunset. The best time to view Jupiter is when the Jovian planet culminates above the southern horizon, at around 11:45 p.m. in early September and 10:00 p.m. at the end of the month. On the night of September 17-18, our Moon comes to within 5 degrees of the giant and its moons, including the four Galilean satellites, all visible through a telescope in the evening.
Despite being more than twice as far away from us, Saturn lies just west of Jupiter from our vantage point on Earth, directly in front of the stars of the constellation Capricornus. You should have no problem finding the dimmer of the two giants: first locate Jupiter, then scan this region of the sky for the second brightest point located 16 degrees to its right, or the height of your closed fist with the thumb pointing up, at arm’s length. Saturn also gets a visit from our natural satellite on the night of September 16-17, one day before its neighbour.
The furthest reflection of sunlight that you can hope to see with the naked eye is from Uranus, at 19 AU. As the planet journeys toward opposition in November, its magnitude approaches the threshold of naked-eye visibility, but the sky must be perfectly clear to have any chance of spotting it in the constellation Aries, more than 60 degrees high due south in the wee hours of morning. Want a tip for observing its diffuse turquoise glow? Uranus sits about halfway along an imaginary line between Orion’s belt and the star Alpheratz, which marks the top-left corner of the square of Pegasus.
At around midnight, extend the imaginary line from Alpheratz to Fomalhaut, a very bright star that just skims the southern horizon in the constellation Piscis Austrinus; at the halfway point is where you’ll find the distant planet Neptune. While optical instruments are recommended for trying to find Uranus, they are an absolute must for observing Neptune, the most distant of our planets, at 28.9 AU.
Are you up for these viewing challenges? Just avoid the nights in mid-September when the Moon’s glow will increase our atmosphere’s brightness, effectively obscuring our view of dimmer objects.
Dive into the stars
The September sky provides an opportunity to dive much deeper into space and time. Remember that light takes a significant amount of time to cross astronomical distances. At the Solar System level, this time ranges from 1¼ seconds (for light to reach us from the Moon) to 4 hours (for it to travel from Neptune). But light emitted from the farthest star we can hope to see with the unaided eye gives us an almost 5,000-year glimpse into the past. That star is VV Cephei, and you can find it by zeroing in on the left edge of the famous Summer Triangle: If you extend the line connecting Altair to Deneb one and a half times, you’ll reach the constellation Cepheus and its iconic yet abstract shape of a house with a pointed roof. Take a look on a fine, moonless night. Can you make out what appears to be a dim, reddish star of fifth-magnitude, located near the middle of this house? Its subtle glow actually masks a binary system whose main component is possibly the largest star visible to the naked eye: a red supergiant thought to be 500 to 1,000 times the size of the Sun!
Beyond the stars is where our gaze gets lost in the cosmic ocean, adrift with random galactic islands, such as the famous Andromeda galaxy, optimally positioned near the zenith in the hush of those September nights. At a distance of 2.5 million light years, it is the farthest—and oldest—light source that our eyes will ever behold.
Take a deep breath and let that sink in!