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Northern Sky

East Bay Moon Crescent/Photo by Stephan Hoglund

Deane Morrison is a science writer at the University of Minnesota. She authors the Minnesota Starwatch column, and contributes to WTIP bi-weekly on the Monday North Shore Morning program through "Northern Sky," where she shares what's happening with stars, planets and more.


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Sky Map June 2020.jpg

Northern Sky: June 20

Northern Sky - by Deane Morrison

Deane Morrison is a science writer at the University of Minnesota.
She authors the Minnesota Starwatch column, and in this feature,
she shares what there is to see in the night sky - in our region.



Northern Sky: May 23 - June 5

By Deane Morrison

Venus has just left the evening sky, and now Jupiter and Saturn are moving in. By mid-month both will be up in the southeast before midnight. Jupiter, by far the brighter planet, shines west of Saturn and leads the ringed planet across the night sky. 
Mars doesn’t quite make it into the evening sky. But it rises earlier each day, approaching midnight from the morning side. By dawn Mars will be a fairly bright dot in the southeast. As for Venus, it reappears in the morning sky this month, but doesn’t climb out of the sun’s foreglow until late June or early July.
If you’re out at nightfall, the brilliant star Arcturus dominates the southern sky. Arcturus, the jewel of kite-shaped Bootes, the herdsman, is the brightest star in the northern hemisphere of sky. However, it’s barely brighter than Vega—the beacon to the east of Arcturus—so they can be considered co-holders of that title.
Between the 28th and 29th, a waxing moon glides between Arcturus and Spica, the brightest star in Virgo, the maiden, which shines about 30 degrees below Arcturus. Whenever Spica is up, you can find it by following the curve of the Big Dipper's handle —always somewhere to the north—to locate Arcturus, then keeping going to find Spica. In other words, “arc to Arcturus, speed on to Spica.” 
June’s full moon arrives at 2:12 p.m. on the 5th. It follows Scorpius and Antares, the scorpion’s bright red heart, across the night sky. Jupiter and Saturn follow the moon that night; between Jupiter and the moon is the Teapot of Sagittarius. 
If you like challenges, look for a very old crescent moon to the lower left of Venus, right above the east-northeastern horizon about half an hour before sunrise on June 19. Then look for an extremely young and thin crescent moon getting ready to set over the western horizon at nightfall on the 23rd. Use binoculars, and see if you can find the Beehive star cluster right below the moon. 
 Summer begins with the solstice, at 4:44 p.m. on the 20th. At that moment the sun reaches a point over the Tropic of Cancer, and an observer from space would see Earth lighted from the Antarctic Circle up to the North Pole and beyond to the Arctic Circle on the night side of the planet.

The University of Minnesota’s public viewings of the night sky at its Duluth and Twin Cities campuses have been curtailed due to the COVID-19 pandemic. For more information, see: 
Duluth, Marshall W. Alworth Planetarium:
Twin Cities, Minnesota Institute for Astrophysics:
Check out astronomy programs, free telescope events, and planetarium shows at the University of Minnesota's Bell Museum:
Find U of M astronomers and links to the world of astronomy at


Star Map - May 2020

Northern Sky: April 25 - May 8

NORTHERN SKI – Deane Morrison
April 25-May 8 2020

In late April and early May, we get to watch Venus sink into the sun’s afterglow. To see our sister planet at its best and brightest, we have to wait for a dark sky, and this time of year the sun is going down later each night. And the longer we wait, the lower Venus gets. 

On Saturday, April 25, a waxing crescent moon appears below Venus and next to Aldebaran, the eye of Taurus, the bull. The next night, Sunday, April 26, a fatter moon appears at about the same level as Venus. Both these solar system objects will be between Betelgeuse, the gigantic red star in Orion, to the lower left, and Capella, the brightest star in Auriga, the charioteer, to the upper right. On the 27th and 28th, the moon moves through the stars of Gemini. On May 1st, the moon is just past first quarter, and it appears above Regulus, the brightest star in Leo, the lion. The moon will also be part of the backward question mark of stars known as the Sickle, which outlines Leo’s head. The moon may wash out the stars of the Sickle, but they’ll be easier to find in several days, after the moon has moved on. Between the 4th and 5th of May, the moon passes over Spica, the brightest star in Virgo, the maiden. 

May’s full moon arrives at 5:45 a.m. on the 7th. It’ll be big and bright, another supermoon. You might want to look for it the night before or the night after, even though it’ll be somewhere around half a day before or past full. If you go out the morning of the 7th to see it at its roundest, be advised that from Grand Marais the moon sets, in the west, at 5:59 a.m. 

The morning sky continues to brighten earlier and earlier, turning into some kind of insomniac’s wonderland. These days you really have to get out by 5 a.m. to see Jupiter, that’s the big bright light in the southeast, and especially Saturn and Mars, before sunlight starts extinguishing them. If you haven’t been following them, Saturn is not far to the left, that is, east, of Jupiter, and Mars is even farther away to the east, and lower. You may notice Mars getting higher from day to day, but the more noticeable change is Saturn and Jupiter moving westward. As the month progresses, the gap between Mars and Saturn gets really wide. 

The Summer Triangle of bright stars is also up in the predawn sky. Above Jupiter and Saturn is Altair, in Aquila, the eagle. Moving up from Altair, we have the brightest star in the Triangle, Vega, in the constellation Lyra, the lyre of Orpheus. And moving down from Vega and a bit east, there’s Deneb, in Cygnus the swan. Also, the Milky Way forms a ribbon stretching from south to northeast. 

The end of April and beginning of May have some good dates for spotting the International Space Station in the morning sky. The sightings run from April 26 to May 1. The most spectacular will probably be the last one, on May 1, when the ISS makes its appearance at 4:32 a.m.,18 degrees above the southwest horizon. It’ll be bright and visible for four minutes, and it gets as high as 60 degrees, or two-thirds of the way to overhead. For a list of the exact dates and times near Grand Marais, search for “spot the station,” click on “sighting opportunities,” go to the map of Minnesota and click on the Grand Portage National Monument icon. 



Northern Sky: March 14 - 27

Northern Sky – Deane Morrison
March 14-27, 2020

The outer planets have all arrived on stage, and in middle and late March they perform the first act of their big morning show. On Sunday, the 15th, look to the southeast for brilliant Jupiter, then just to the west for a little reddish dot. That, of course, is Mars. East of Jupiter and lower in the sky is Saturn, which is following Jupiter as it approaches Mars. On Wednesday, the 18th, Jupiter will have moved noticeably closer to Mars and a waning, but hefty, crescent moon will be hanging right below them. The next morning, Thursday, the 19th, Jupiter and Mars are even closer and a slightly thinner moon joins all three planets when it rises at 5:26 a.m. On the 20th, Jupiter passes a mere 0.7 degrees—slightly more than a moon width—above Mars. And the moon rises at 6 a.m.
So if you’re up, consider going outside by 6 a.m. on any of those days—the 18th through the 20th. You’ll see three planets, two of them in the process of passing each other, and maybe a moonrise, too. All in a dark sky. That doesn’t happen every day.
As we near the end of the month, Saturn closes in on Mars. On the morning of Saturday, the 28th, the ringed planet will be at about the same height as the red planet, getting ready for the second planetary pass in less than two weeks. By then, Jupiter will have removed itself a respectful distance to the west.
In the evening sky, another approach is in progress. The Pleiades star cluster and Venus are moving closer together as Earth’s orbital motion drops the Pleiades down toward the sunset horizon and Venus’ orbit carries it higher. Venus is unmistakably bright. The Pleiades, or Seven Sisters, are a slightly blurry bunch a little west of and lower than the bright star Aldebaran, the eye of Taurus, the bull. On Saturday, March 14th, the Pleiades will be high above Venus. On the 27th, the star cluster will be closer to Venus than to Aldebaran. Make sure your binoculars are in good working order as Venus and the Pleiades get ready to meet in the first few days of April. And also, just enjoy Venus. From now until well into April, it’s visible for a good three and a half hours after sunset.
East of Aldebaran, the hourglass form of Orion is tilting to the west as it begins its annual exit from the evening sky. We’re also getting into the last few weeks when Sirius, the brightest star in the night sky, is high enough for us to enjoy its full radiance; look for it low in the south-southwest at nightfall.
High in the southeast, Leo, the lion, is prancing into view. Leo is a two-part constellation. Its western section is the Sickle, a backward question mark of stars whose point is the bright star Regulus, the lion’s heart. East of the Sickle is a triangle of stars marking the lion’s hindquarters and tail. Leo follows the winter constellations across the sky and makes a very recognizable harbinger of spring.
And speaking of spring, it arrives on the 19th, at 10:50 p.m. At that moment the sun crosses the equator and enters the northern sky, and an observer in space would see Earth lighted from pole to pole. Also, since the fall equinox it’s been the case that as you traveled northward, the days got shorter. At the spring equinox that reverses, and the days get longer as you head north, all the way to the North Pole, which now has, officially, 24-hour daylight for the next six months.



Northern Sky: February 29 - March 13

NORTHERN SKY – Deane Morrison
February 29 – March 13, 2020
In March, the action in the predawn sky really picks up. The month opens with Mars, Jupiter and Saturn forming a straight line, in that order from right to left, above the southeastern horizon. On the 4th, the three planets are spaced almost evenly apart.
But that neat arrangement soon gives way as the planets switch positions. Jupiter and Saturn are about to leapfrog past Mars, and all because Mars, being the closest to the sun, orbits the fastest. 
What’s happening is that Earth’s orbit is carrying us eastward and thus toward all these planets, making them move higher and westward. But Mars’s own orbit carries it much more rapidly eastward against the background of stars than the orbits of Jupiter and Saturn carry those planets.
As a result, Mars resists the westward movement imposed by Earth’s orbit and appears to sail eastward toward—and past—the two giant planets. Actually, though, Mars is mostly holding its own as Jupiter and Saturn sail past it on their westward journey. 
Have a look on the 18th, when Jupiter and Mars make a close pair while a crescent moon hangs right below them. The next morning, Jupiter and Mars will be closer yet and the moon will now appear below Saturn. On the 20th, Jupiter passes a mere 0.7 degrees—slightly more than a moon width—above Mars. On the 31st, the ringed planet passes about a degree above the red planet. In April the new lineup will be, from right to left, Jupiter, Saturn and Mars, with the gap between Mars and the other two planets rapidly widening.
In the evening sky, a young moon comes out below Venus on the 26th and 27th. And don’t miss the show as the brilliant planet and the Pleiades star cluster approach each other in the last week of March. Mark your calendars for April 2nd and 3rd, when Venus glides, spectacularly, in front of the Pleiades. Be sure to have binoculars handy.
March’s full moon shines the night of the 9th. It will be big and bright, though more than six hours past full when it rises that evening.
Spring arrives with the vernal equinox at 10:50 p.m. on the 19th. At that moment the sun crosses the equator into the northern sky and Earth will be lighted from pole to pole.

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:
Duluth, Marshall W. Alworth Planetarium:
Twin Cities, Minnesota Institute for Astrophysics:
Check out astronomy programs, free telescope events, and planetarium shows at the University of Minnesota's Bell Museum:
Find U of M astronomers and links to the world of astronomy at


Sumitted by Deane Morrison

Northern Sky: February 15-28, 2020

Deane Morrison is a science writer for the University of MN and authors the Minnesota Starwatch column, and in this feature she share what there is to see in the night our region.

Northern Sky by Deane Morrison for February 15-28, 2020.

In the second half of February, it doesn’t matter if you’re looking at the post-sunset or the predawn sky; you’re going to see upwardly mobile planets.
In the evening, Venus is still blazing away as a gorgeous evening star. If you look to the west, you won’t have any trouble finding it. As it climbs farther above the horizon, it sets later. Venus is currently up and bright for more than three hours after sunset.
No bright stars are near Venus, but a young crescent moon stops by on the 25th, 26th, and 27th. The moon is way below Venus at first, but on 27th, it’ll have moved to about the same level as Venus. The bright winter stars are in the south after nightfall, so if the night is clear and moonless, don’t miss the chance to see them if you haven’t already. But even Sirius, the brightest of all, can’t match the candlepower of our sister planet. As they say, location is everything.
The three bright outer planets are now assembling in the predawn sky. They’re all fairly low in the southeast, with Mars leading the way—that is, it’s the highest and farthest to the west. Next comes brilliant Jupiter, and finally Saturn. Late in the month, when Saturn is high enough to be easily visible, the three planets form a straight line with Mars at the upper right end, Jupiter in the middle, and Saturn at the lower left end. This assembly sets the stage for some of the closest approaches between these three planets that we’ll ever see, as both Jupiter and Saturn pass Mars.
What’s happening is, Earth’s orbital motion is carrying us closer to all three planets; this pushes them higher and farther west each day. But they don’t move west at the same speed. As they orbit the sun, all the planets move eastward against the background of stars. Because Mars orbits eastward at great speed, it resists Earth’s westward push. But Jupiter and Saturn orbit sluggishly, and so Earth’s motion is pushing them westward, toward Mars, rather fast. In just a few weeks we’ll see first Jupiter, then Saturn, catch up to Mars and pass it.
Also, since Jupiter’s orbital motion eastward is faster than Saturn’s, it’s slowly closing in on the ringed planet. We can watch this slow approach until December, when Jupiter and Saturn pass each other in the evening sky.
In the days leading up to the new moon on the 23rd, a waning the moon plunges through the morning sky, toward the rising sun. On the 18th, the moon passes right in front of Mars. The red planet disappears behind the moon’s bright leading edge, which is to say, behind the lighted crescent. Mars reappears behind the moon’s dark edge, but not until after daylight. If you want to see Mars disappear, get out by 6 a.m. and bring binoculars. This won’t be the most spectacular lunar occultation, as these eclipse-like events are called, because Mars is still pretty far away and small, so don’t feel bad if you miss it.
The next morning, the 19th, a thinner crescent moon will be staring right at Jupiter, and on the 20th an even thinner moon rises below Saturn. Try to catch those two about an hour before sunrise, before Saturn gets washed out.
Also, between the 24th and 25th, Mars glides right above the star that marks the lid of the Teapot of Sagittarius. Meanwhile, Jupiter pretty much stays put below the softly curving Teaspoon of stars. If you’ve never seen these features of Sagittarius, these planets can be your guide.


Starwatch November 19

Northern Sky: November 23 - December 6

Northern Sky by Deane Morrison

November 23 to December 6, 2019

As we move into the final month of the year, the planets are at play in both the morning and evening skies.
In the predawn hour, Mars and Mercury are both in the east-southeast until the end of November and into December. So is Spica, the brightest star in Virgo, the maiden. These three objects form a diagonal line above the horizon with Spica highest, Mercury lowest, and Mars, of course, in the middle. You might want to look on Sunday, November 24, when a thin crescent moon hovers to the left of Mars.  On the 25th, the moon will be thinner and just above the horizon, below Mercury.
As the days go by, Mars’s two companions abandon it. Spica keeps climbing higher, thanks to Earth’s orbital motion. But Earth's motion has a relatively weak influence on Mercury's comings and goings because it's an inner planet and much speedier than Earth. When it pops into the morning sky, it’s coming out from a trip between Earth and the sun. The transit of Mercury across the face of the sun on November 11 was also a transit, officially, out of the evening sky and into the morning. But it soon drops down again, as it starts its next trip around the far side of the sun. So as Spica climbs away and Mercury drops away, Mars gets doubly deserted. 
In the evening sky, Jupiter and much brighter Venus pass each other in the southwest on the weekend before Thanksgiving, November 23 and 24. The two planets come as close as 1.5 degrees, or about three moon widths. Try to catch them about 40 minutes after sunset. On Thanksgiving evening, a thin crescent moon of the next cycle will be right above Venus, and Jupiter will be below it to the right. This threesome will be impressive if you can catch it before Jupiter sets. If not, that’s okay; Venus and the moon always make a nice pair. 
On Friday, November 29, the moon hangs below and left of Saturn. Saturn hasn’t been especially bright this year, so if you haven’t found it, this moon will guide you. Like Jupiter, Saturn is falling toward the sunset as Earth leaves it behind in the orbital race. The ringed planet also gets a visit from Venus, in the second week of December. It'll be fun to watch these two planets approaching each other.
If you’re out at nightfall, look to the north to see the Big Dipper sitting right above the horizon. When it’s low like this, you can observe the double star at the bend of the dipper’s handle through binoculars without straining your neck too much. Also, find the pointer stars at the far end of the bowl, that is, away from the handle; these point to Polaris, the North Star. Then see if you can spot a lazy triangle of stars to the upper right of the pointer stars; these mark the head, and rather pointed nose, of the Big Dipper’s home constellation: Ursa Major, the great bear.  
Bracketing Polaris are two bright stars, both at about the same distance from Polaris. To the left, in the northwest, is Vega, in the Summer Triangle of stars, and to the right, in the northeast, is Capella, in the winter constellation Auriga, the Charioteer. Hanging from Polaris is the Little Dipper. Its midsection is dim, and it’s about all there is to see in the constellation Ursa Minor, the little bear. But since the Latin name Ursa Major is feminine, and Ursa Minor is so close by, Ursa Minor could well be regarded not just as a little bear, but as a bear cub.



Starwatch Map Nov 2019

Northern Sky: November 9 - 22

Northern Sky by Deane Morrison
Nov. 9-22 2019
Now that we’re back on Standard Time, nightfall comes early. The switchover sucked an hour of darkness from the morning sky, so now we have to get out really early to watch the morning stars and planets. Sure, the sun has been rising later each morning since the switchover, but we won’t get all that darkness back again until December 17. The sun will keep coming up later for the rest of December and into January, but on January 6 it starts coming up earlier. 
If you’re out an hour before sunrise between November 9 and 13, you can watch Spica, the brightest star in Virgo, the maiden, climb in the eastern sky and slide past Mars. Spica and Mars are closest on the 10th. Then on the 11th and especially the 12th, the two are at virtually the same altitude, with Spica on the right. Spica and the other stars go right on moving higher and westward, but Mars’ orbital motion eastward keeps it from pulling away from the rising sun as fast as the stars do. So Mars will be low and dim for a while, but in less than a year it’ll be the star of the evening show.
Over in the west, the sky is waiting for the next “evening star” to climb into prominence. That would be Venus. Venus is slowly pulling itself out of the setting sun’s afterglow. Meanwhile, Jupiter and Saturn are sinking toward it as Earth leaves them behind in the orbital race. By the 22nd, Venus and Jupiter will have drawn close together, very low in the southwest, with Venus, the brighter planet, below Jupiter, getting ready to pass it. As they approach each other and pass, the trick is to catch the planets when it’s gotten dark enough to see them but before they drop too close to the horizon. When Venus and Saturn pass next month, it’ll happen a little higher in the sky.
At nightfall on November 10th, the waxing moon will be above the eastern horizon. At about the same altitude, off to the left, is the bright star Capella, in the constellation Auriga the charioteer. Later in the evening, when the bright winter constellations are all up in the east, Capella will be at the top of the heap. As for the moon, to see it closest to full, look westward on the morning of the 12th, at least a half hour before it sets at 6:51 a.m. 
On Monday the 11th, the sun rises with a tiny round, black dot on its face. That's Mercury, making a rare transit, which occurs when Mercury passes between Earth and the sun and we see Mercury crossing the face of the sun. Mercury passes between us and the sun three or four times a year, but because the plane of its orbit is tilted seven degrees from Earth’s, its orbit usually carries it above or below the sun from our point of view. If our two planets orbited in the same plane, we’d see transits during every passage of Mercury between Earth and the sun. For us to see a transit during a passage, Earth must, at that moment, be at or very near one of the two points where its orbit intersects the plane of Mercury’s orbit. That puts both Mercury and the sun in our line of sight, and so the planet appears to crawl across the sun’s face. From Minnesota, the transit will be under way at sunrise, but it lasts until 12:04 p.m. Don’t watch it without proper eye protection, especially when some websites will stream it live. Just search online for “live webcast Mercury transit 2019.” 


Northern Sky Map.  Courtesy Deane Morrison.

Northern Sky: August 31 - September 13

by  Deane Morrison
August 31 - September 13, 2019

Over Labor Day weekend, the moon is a young sliver that sets before or shortly after nightfall. Each night it moves farther eastward, on its monthly tour of the sky. It’s waxing brighter now, and won’t begin withdrawing from the evening sky until after the full phase in mid-September. So if you’re an evening star watcher and you go out during the two weeks after Labor Day, you’ll probably be seeing a lot of the moon.  

As for the stars, this month the Summer Triangle is high in the south, in prime position for evening viewing. The brightest of the three stars, Vega, in the constellation Lyra, the lyre, is just a hair less bright than the star you may have noticed slowly sinking in the west. That’s Arcturus, the anchor of Bootes, the herdsman, and the brightest star in the northern hemisphere of the sky. In the east, the Great Square of Pegasus, a fall constellation, is climbing into prominence.

Every year the stars and constellations are the same, but the planets move around. This summer, we’ve been treated to the sight of Jupiter and Saturn in the south after nightfall—and Jupiter is so bright, it comes out in the twilight, well before Saturn. On the 5th, the waxing moon hovers near Jupiter and almost directly above Antares, the brightest star in Scorpius. At 10:10 that night, Thursday, the 5th, the moon reaches first quarter phase. When the moon is at a quarter phase, the features of the lunar surface cast the deepest shadows, and this makes them stand out in sharpest relief. So this is a good night to explore the lighted part of the moon with binoculars. You don’t have to wait till after ten o’clock, of course; the moon is essentially at the first quarter phase all evening.

On the 7th, the moon will be bigger and brighter, this time closing in on Saturn. You may need binoculars to see it, but the star a few degrees below and right of the moon is the lid of the Teapot of Sagittarius. The Teapot will be somewhat washed out by moonlight, but if you haven’t seen it yet and would like to, that’s where to look for the lid, at least. On the 8th, the moon will be east of Saturn. On the 10th, the moon will be in the middle of the constellation Capricornus, the sea goat. Capricornus is chevron-shaped, and it’s one of the autumn water constellations. It has no bright stars, so it’s hard to find. But if you note where the moon is on the 10th and then find a star chart, that’ll vastly improve your chances of finding this dim constellation of the zodiac.

Three days later, on the 13th, we get a full moonrise. In Grand Marais, the moon rises at 7:36 p.m., four hours before it becomes perfectly full. Because it’s the closest full moon to the equinox, it qualifies as the harvest moon. The harvest moon got its name because near the time of the fall equinox, the moon can rise less than 30 minutes later from night to night as it goes from almost full to full to a couple of days past full, as opposed to forty or fifty minutes later from night to night near the solstices and more than an hour later from night to night near the spring equinox. The harvest moon gave extra moonlight to farmers working late to harvest their crops before they either froze or spoiled in the field. The harvest moon effect isn’t always as strong as the numbers I just gave would imply, but even so, this nickname for a full moon has a pretty solid basis. 


Star Map July 2019 by Deane Morrison

Northern Sky: July 20 - Aug 2

NORTHERN SKY - Deane Morrison
July 20 - August 2, 2019

Today, in honor of the 50th anniversary of the moon landing on July 20, I’d like to talk about how it happened and why it was scientifically important.
On July 20, 1969 the moon was a waxing crescent in the evening sky. The Apollo 11 astronauts had a landing site all picked out near the border with Earth’s shadow, where it was just past sunrise. As the computer aboard the lunar module, the Eagle, guided them down, Neil Armstrong and Buzz Aldrin watched the moonscape go by below them. At about 33,000 feet, an alarm suddenly went off. Nothing in their training had prepared them to figure out what it was. They spent several minutes trying to identify it before Mission Control finally told them not to worry; it was just the Eagle’s computer reacting to being overloaded.
While the astronauts were chasing this wild goose, the lunar module used a lot of propellant fuel, and when the fuel tank dipped to half full, the propellant started sloshing around and knocking the module every which way. The sloshing also set off a “low fuel” alarm prematurely, and so the crew thought they had less time to land than they actually did.
These issues also kept Armstrong from checking out the landing site visually. At about 2,000 feet, he finally did, and he saw that the site was a boulder field—not smooth ground at all. At about 540 feet, Armstrong took manual control of the spacecraft. Despite the disruptions, he was able to guide the lunar module past the boulder field and set it down with just 20 seconds left before they would have been forced to land wherever they were or abort the landing.
Apollo 11 brought back 46 pounds of moon rocks. In all, 842 pounds of moon rocks came back on Apollo spacecraft. The oldest are 4.5 billion years old, same age as Earth. They indicated that the moon, like Earth, was also once covered by an ocean of magma. But unlike Earth, which has an iron core, the moon has very little iron and came from the same stuff as Earth’s mantle. These findings support what was once a radical idea: that the moon didn’t condense out of the same cloud of solar system material as Earth, nor was it captured later by Earth, but it formed when another planetary body collided with an infant Earth that was still molten. The collision knocked out material from Earth’s mantle, and perhaps from the other body, and this material coalesced to become the moon.
Also, moon rocks record impacts from collisions, and the number of impacts apparently spiked around 700 million years after Earth and the moon formed. An explanation for why so many more objects should have been flying around the solar system and hitting the moon—and Earth, of course—at that time is part of a leading theory of how the solar system formed. Simply put, it says that early on, gravitational interactions between Jupiter and Saturn destabilized the entire solar system and pushed the orbits of Saturn, Uranus and Neptune farther out. This changing gravitational landscape disrupted the orbits of numerous small bodies far from the sun, collectively called protoplanetary debris, and scattered them throughout the solar system. The result was an outbreak of collisions that lasted until most of the debris was cleared. And this outbreak, called the late heavy bombardment, left its signature in the spike of impacts imprinted in the moon rocks.

Scientists are still debating exactly how the moon formed and what the moon rocks tell us. But by any measure, these rocks are among the most valuable items of all time.
Deane Morrison writes the Minnesota Starwatch column for the University of Minnesota’s Minnesota Institute for Astrophysics.