EARTH — The Autumnal Equinox occurs this Saturday, Sept. 23. This event marks the official end of summer for the northern hemisphere, and ushers in the autumn, with cooling temperatures, clearer night skies, and amazing astronomical viewing opportunities.
The equinox occurs when the sun appears directly over the equator. Our planet is lit directly from pole to pole, and we experience days with equal length of daylight and nighttime.
The sun changes position in our skies due to the tilt of the Earth. During the summer months in the northern hemisphere, the north end of the planet is tilted more toward the sun, presenting us with more direct sunlight, causing warmer temperatures and longer days.
The tilt of the Earth never changes. But as Earth orbits the sun, the angle of sunlight upon our globe changes. As we make our way around the sun, the angle affects the amount of sunlight to fall upon any given point upon the globe.
But each equinox is truly the great equalizer. Every point on Earth theoretically experiences twelve hours of sunlight, followed by twelve hours of night. The sun is directly above the equator, and it lights precisely one half of the globe from north pole to south.
From the moment of the equinox, the sun will appear to drift south, bathing the southern hemisphere in more light, and our end in less. The daylight hours will get shorter, and the nights colder.
The sun is directly above the equator for only a moment, at precisely 12:50 a.m. Mountain Standard Time on the 23rd of September. From that moment on, it appears to continuously move south.
Many ancient civilizations observed the movement of the sun, and precisely calculated the dates, times, and positions of the equinox events. These were the results of countless years of observations of the sun, as it makes its motions across our skies.
From this point on, we will have more dark sky hours to work with, as the daylight hours wane and shorten as we approach winter.
This weekend will provide a perfect opportunity for us to find Mercury in the morning sky. It appears at its highest position from the horizon this weekend, rising a full 86 minutes before the sun.
Go out in the predawn hours, and look to the east. Find brilliant Venus, already high in the sky, more than three hours ahead of the sun, and two hours in advance of Mercury.
Look below Venus, to the left, above the point on the horizon where the sun will rise. There you can spot tiny Mercury, as it appears furthest from the rising sun in the early morning sky. This weekend provides the last best chance to view Mercury at its highest, in a darker sky.
From this point on, Mercury will begin to descent, lower each morning, as it drifts further off from Earth, in its rapid orbit. Mercury is gaining distance on us, and is already further off than the sun. As it speeds away, it will fade into the distance as it descends toward the glare of the sun.
As the moon moves through the sky this week, it will have a brief and distant encounter with Saturn. On the evening of Tuesday, Sept. 26, the bright moon will be to the lower right of the ringed planet.
Though Saturn is bright right now, it lies among some pretty brilliant stars, and locating and identifying it can be a little difficult. But the moon on Tuesday evening will guide your way. The moon will lie only three degrees below Saturn at its closest at around seven o’clock that night.
Brilliant Jupiter will rise two hours later, as it continues to brighten, as Earth is getting nearer the largest planet.
As promised, we will check in on the position of Neptune this week. Neptune is the furthest planet, and unviewable with a naked eye. Both Uranus and Neptune can not be easily seen from Earth due to their great distances. Bright Jupiter lies between the two right now.
As the moon moves through the sky, encountering the visible planets each month, it also passes the ones we can’t see. On Thursday night, Sept. 28, the moon will be only two degrees south of Neptune, the most distant planet in our solar system. Though you can not see Neptune without a telescope, the moon will mark the position in the sky where Neptune resides. Off to the east will be bright Jupiter, and beyond that, further east, is the position of Uranus, which we will mark next week.
The dawn of October comes next weekend, and on the night of Sunday, Oct. 1, the moon will have move to take up a position near Jupiter. Go out next Sunday night, and find the moon, now just past full. To the lower right, Jupiter will lie just three degrees below the moon. They will be at their closest shortly after their rising.
The moon will appear on the horizon ten minutes after eight. Ten minutes later, Jupiter will appear below it. Watch as the pair rises together, and they are at their nearest an hour later. Keep an eye on the pair as they rise, the distance between the two will increase, until Jupiter and the moon appear almost directly overhead, the gap between them larger than when they arose, as the sun begins to lighten the sky in the morning hours.
Brilliant Venus is always a beautiful sight in the morning sky. It continues to shine at about its brightest right now, and rises a full three hours before the sun, so it has plenty of time to dazzle our eyes in the dark, predawn sky.
With our dark skies here, they only need to be clear, so keep your eyes on the skies from sunset to sunrise.
Thanks for the positive feedback about our featured columns, and your continued interest in astronomy. If you have any questions or article requests, contact us at spacescape@rocketmail.com, or follow us on Twitter @ColoSpacEScapE for updates and additional viewing opportunities.
Astronomical times and distances of naked-eye objects for this weekend:
Sun Set = 6:53 p.m.
11 minutes earlier than last week
93.270 million miles from Earth
183,237 miles nearer than last week
1.235 million miles nearer than its furthest in July
Mars Set = 7:36 p.m.
16 minutes earlier than last week
43 minutes after the sun
235.508 million miles from Earth
997,643 miles further than last week
184.783 million miles further than its nearest in Dec.
Jupiter Rise = 8:49 p.m.
28 minutes earlier than last week
389.927 million miles from Earth
7,214,264 miles nearer than last week
163.557 million miles nearer than its furthest in April
Moon Set = 12:44 a.m.
66 minutes later than last night
230,555 miles from Earth
2,971 miles nearer than last night
17,938 miles nearer than last week
Nearest is 221,557 miles
Furthest is 252,665 miles
Venus Rise = 3:29 a.m.
12 minutes earlier than last week
44.312 million miles from Earth
4,428,981 miles further than last week
17.474 million miles further than its nearest in Aug.
Saturn Set = 4:28 a.m.
30 minutes earlier than last week
825.001 million miles from Earth
4,572,489 miles further than last week
10.422 million miles further than its nearest in Aug.
Mercury Rise = 5:21 a.m.
3 minutes earlier than last week
86 minutes before the sun
93.351 million miles from Earth
17,910,977 miles further than last week
34.980 million miles further than its nearest in Sep.
Sun Rise = 6:47 a.m.
6 minutes later than last week
93.257 million miles from Earth
13,247 miles nearer than last night
184,131 miles nearer than last week
1,248,704 miles nearer than its furthest in July.
Full Moon occurs on Friday, September 29, at 3:57 a.m. We’ve lost 17 minutes of daylight since last week, and 71 minutes since last month, and a full two hours 38 minutes since the June Solstice. Our nearest planetary neighbor currently is Venus. It is 48.945 million miles nearer than the sun.
Note: Times are local Mountain Time. Actual “sundown” is about a dozen minutes earlier than calculated “sunset”. Along the front range, differing times vary depending on your distance from the mountains.
William J. Bechaver is the director of SPACE • Spanish Peaks Amateur Cosmos Enthusiasts, the premier Astronomical Society for Southern Colorado and Northern New Mexico.
