Venus, Mars and Saturn are in the homestretch of a long developing planetary alignment. Currently appearing in a nearly straight line stretching away from the setting sun, they will slowly draw together in the western evening sky as July progresses, finally forming a small triangle on Aug 9th before fading into the sunset. And if you watch this dance unfold you’ll be able to see first hand how the entire Earth and our fellow planets go hurtling around the sun through their orbits.
I’ll show you how to see the celestial racetrack we’re on, how to see the night sky in three dimensions, as the planets race around it. First, find yourself a good view to the west and south, and go out an hour after sunset. Low above the western sunset and unmistakably bright is the planet Venus. To the upper left is the background star Regulus in the constellation Leo. Continuing on are red Mars and yellow Saturn. After Saturn comes the bright blue star Spica in Virgo and finally massive orange Antares in the heart of Scorpio, low in the south.
Take your arm and sweep it along the path from the set sun through the planets out to Spica and around to Antares. That is the plane of the Ecliptic, the surface of a great "racetrack" that the Moon and planets move along. The Ecliptic passes through the 12 constellations of the Zodiac, which is why they are featured in astrology.
As we here on Earth move along our own path around the Sun, the planets move against the background stars as well as in relation to each other. When you look to the west after sunset, you are looking backwards along the "track" as the Earth moves east. Imagine you’re looking out the back window of a car speeding around a huge circular track, with the Sun at the center. The stars are the "grandstands" off in the distance, and our view of them is constantly changing as we circle the track. Venus, Mars and Saturn are all "cars" on the track, speeding along behind us.
Venus, being closer to the sun and therefore moving faster, is coming around from behind to lap us. Mars and Saturn, further out and slower, have already been lapped by us and are falling back. But like a slow pass on the interstate where the headlights slowly fade, they remain in view. The relative closeness of Venus and Mars to us will be easily visible as they move quickly against the background stars while they keep pace around the curve. Venus will reach and pass Regulus only a week from now on July 9th-10th. You will be able to see the distance change daily. At the same time Mars will greatly close the gap with Saturn. The Moon will join in between July 13th and 17th. To top it all off, fast little Mercury will swing around from behind the Sun to lap all of us, and should be visible for a week or so beginning around July 23rd.
Saturn is much, much farther out from the Sun, and moves accordingly slower. While Venus takes 224 days to orbit the Sun, and Mars just under 2 years, Saturn takes roughly 30 years to complete one orbit. Compared to the "cars" of Venus, Earth and Mars whizzing around the inner track, Saturn is like someone jogging along the far outer wall. It’s moving so slowly that as we pass by, Saturn falls back at about the same speed as the background grandstand of stars. Take note of how Venus and Mars hold relatively steady night to night while Saturn and the stars sink into the west behind them. Venus, now appearing in front of Leo will instead be in front of Virgo and will graze Spica on Aug 31st.
So over the course of July and early August as you look out the back of our "car", Venus is closing in to pass us on the inside, while Mars slowly falls back and Saturn and all the stars fade around the bend of the curve. And right before Venus overtakes us and is lost in the glare of the Sun, Mars and Saturn will line up far behind it back around the curve, reaching their closest alignment on Aug 9th and 10th. And as you watch the planets change positions daily, remember the vast spread of distances involved. Venus will be 75 million miles back, Mars 200 million behind, Saturn 1 billion miles and the stars dozens or hundreds of light years away. And the Earth quietly continues on down the track, in a race that never ends.
Enjoy the backyard astronomy!
Update: Wow, this is my very first time on the rec list, thanks everyone! I wish I could stick around longer, but my wife's got me cleaning the basement for a party tomorrow so I have to run. I'll check in when I can. Thanks again and have a great summer skywatching.
Update 2x: To complete the visual imagery:
So as we're driving down this wide celestial track, the Moon is this strange little car that circles around us. Looking East at dawn it passes us on the right, changes lanes to the left and drops back behind us.
Looking backwards at dusk it changes back to the outside lane, crossing right to left as you look west. And then it passes us again. The entire time it does this it stays on the "racetrack", in the plane of the Ecliptic.
The Full Moon occurs when it's in the outside lane and fully in the sunlight.
An eclipse is when it's in the inside lane between us and the Sun, and because the Sun is also in the plane, the Moon can cross directly in front and blot out the Sun for a minute as it drops back again.