Why do we always think of North as up?

Hi! It might be a dumb question but it’s been in my mind for a while. We are convinced that North is up and South is down because that’s the way maps have been for many many years, but we don’t really know which way is actually up, it could be east or northwest, etc, right? Because there isn’t a real orientation/position in space, there’s no fixed up or down, but... doesn’t the way the Earth rotate determine in a way which way is up? How do those two things related to each other? Or is there no connection at all? Thank you!
 "The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit:  NASA

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit: NASA

You’re right that the way we draw our maps with North pointing up and South pointing down is largely arbitrary, and indeed there are a number of maps with the Southerly direction at the top rather than at the bottom, and they’re good fun to look at However, there are good reasons to say that a Northerly or Southerly direction should be “up”, and these reasons extend beyond just the rotation of the Earth.

The rotation of the Earth is a good starting place, though - the rotation axis of the Earth goes more or less through the North and South magnetic poles of the Earth. The magnetic North & South poles wander a little, so some years they’re closer to the rotation axis than others. Fixing the rotation of the Earth as a cardinal direction makes good sense, and is what we’ve done - East and West point 90 degrees from North and South.

There’s one more reason to put North as up, and it’s a physics convention. Most of the time, when we’re talking about rotation, we say that the direction of the rotation axis is actually just in one direction, rather than having to indicate both North and South. If we do this, it allows us to encode both the axis of rotation, and the direction of rotation at the same time. The way we determine which of North or South should be “the direction”, we use what’s called the “right hand rule”. You curl your fingers in the direction of rotation, and your thumb points in the direction of the rotation axis. In the Earth’s case, we rotate towards the East, so your thumb will point in the direction of North.

 A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits:  NASA

A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits: NASA

However, if you’re thinking of orientations beyond just the Earth’s own rotation, while it’s true that there’s no way to set an entirely objective zero point from which to measure other positions, and a sphere doesn’t have much intrinsic orientation to it, we can still do relative positions pretty well. And on the scale of our solar system, we have a pretty solid alignment going on. All the major planets in our solar system trace oval paths around the Sun as they go about their respective years. Not only do they orbit around the Sun in the same direction, they all tend to point their rotation axes in the same direction (notable exceptions here are Venus and Uranus). On top of all that, the ovals are almost perfectly aligned in a flat plane. If we take our same physics convention and use the rotation of the planets around the Sun to tell us which direction we’re going to point up, our Planet Earth based North is more or less pointing in the right direction. Our planet’s spin is not perfectly aligned with the “up” out of the solar system, but tilted by 23 degrees, a feature of our planet responsible for our seasons. This tilt is why many globes are set at an angle - they’re mimicking the tilt of our planet relative to the “up” defined by our solar system.

So the North is up convention is partially mapmakers, partially the spin of our Earth, and partially physics notation, but there are definite ties between all of them.


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Where is the Moon's water?

If there is water on the Moon, will it be on the surface or will it be within the ground?
 This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

There is water on the moon! As we’ve outlined before, it’s somewhat tricky to keep water on the surface of the moon, because the combination of heat and particles from the Sun, a lack of an atmosphere, and no magnetic field means that it’s pretty hard to keep water that’s exposed to sunlight from evaporating away into space.

What that means is that if you want to have water persist anywhere on the Moon, it has to be sheltered from the Sun somehow. An easy way for this to happen is at the poles, where some craters are deep enough that the Sun’s rays never reach into the bottom of the crater. Places like this are called “cold traps”, because it can trap material in a solid form that would otherwise escape if it weren’t so cold.

Near the south pole of the Moon in particular, we found frost in some of these deep dark places. This frost makes the surface more reflective than it would be if there were only rock sitting around in those craters - so the coldest places also wind up being more reflective if you’re bouncing light off of ice. This particular study is careful to note that we’re not seeing frozen pond-style pools of water, but more like the frost that builds on the outer edges of leaves in fall.

But craters aren’t the only places that water ice could hide - we have a sneaking suspicion that the Moon also has tunnels woven under its surface. The Moon had a surprisingly long era of volcanic activity in its younger years, and where there are lava flows, you can wind up with lava tunnels. We are pretty sure that the moon has these. We see them most easily as they collapse, because then you get a snake-like pattern of collapsed ground, twisting its way across the surface as a series of giant potholes.

 These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

Every so often, there’s a more isolated cave-in, giving us a glimpse into a sublunar cavern - a deep shadow cast into the depths catches the eye and the imagination. If water had accumulated in these hidden tunnels, they would also be relatively protected from evaporation. However, it’s one thing to have a plausible place for water, and another to find it for sure in those places. Lava tunnels are an extremely appealing place for water, though - because they’re also an appealing place to put a human base on the Moon. While we don’t have to worry about humans evaporating, any shelter from intense heat and cold helps us as well. So if there were also water down there, they’d be a great place to put an inhabited base.

You can definitely also wind up with watery molecules bound up in the rocks themselves. A recent study suggests that instead of having lots of water ice hanging around, the Moon may have a lot of hydroxyl, which is one hydrogen and one oxygen bound to each other, rather than the two hydrogens and one oxygen that make up your standard water molecule. Hydroxyl binds easily to other things, so it can wind up binding itself to minerals in the earth - you can extract it and create water, but it’s more energy intensive than just having water lying around.

So the true answer is that there’s going to be a mixture of places where water will be found - on the surface in sheltered places, possibly in underground tunnels, and some not-quite water bound up in minerals!


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How Do Black Holes Get Started?

How do black holes get started?
 This artist's impression shows the orbits of three of the stars very close to the supermassive black hole at the centre of the Milky Way. The position of the supermassive black hole is marked with a white circle with a blue halo. Image credit: ESO/M. Parsa/L. Calçada

This artist's impression shows the orbits of three of the stars very close to the supermassive black hole at the centre of the Milky Way. The position of the supermassive black hole is marked with a white circle with a blue halo. Image credit: ESO/M. Parsa/L. Calçada

Originally posted on Forbes!

It depends on how big the black hole is! If you’re dealing with a small black hole, then we have a pretty good understanding of how the black hole forms. The smallest astrophysical black holes are objects that form during the catastrophic explosions of dying, large stars.

These stars contain so much mass that when they begin to explode in a supernova, the shock wave of the explosion can ricochet down into the core of the star, compressing it down to the point that the object in the center of the star becomes too dense for electrons to hold atoms apart (the end point for a white dwarf), further down until the star becomes too dense for neutrons to hold each other apart (the end point for a neutron star) and after that point, the object becomes so dense that even light can’t escape it. At that point, it seems logical to assume that the object itself will continue to press itself inwards, subject to its own ever-increasing gravity, until it takes up no more space than an infinitely tiny point - a singularity.

Left behind, far outwards, is the contour in space which marks the threshold of no return for light - if light travels closer than this horizon, it’s not coming back. This spherical contour surrounding the black hole is known as the event horizon, and often this whole region of space is called the black hole, as it’s a region where the existence of the black hole is the most important thing around.

 This artist’s impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300. The black hole has a mass of about twenty times the mass of the Sun and is associated with a Wolf–Rayet star : a star that will become a black hole itself.  IMage credit: ESO/L. Calçada/M.Kornmesser

This artist’s impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300. The black hole has a mass of about twenty times the mass of the Sun and is associated with a Wolf–Rayet star : a star that will become a black hole itself.  IMage credit: ESO/L. Calçada/M.Kornmesser

The black holes formed this way are a few times larger than our own Sun, made out of stars at least eight times larger than our own Sun. When you want to create black holes which are larger than that, like the supermassive black holes at the centers of galaxies, the situation has to be slightly different. These black holes are many, many times larger than our own Sun - millions to billions of times more massive than the black holes which form from individual stars. How did they form? If we want black holes to grow to this size, we’re going to have to give them a lot of time.

Fundamentally, though, we still need to take a lot of mass, and compress it somehow down to a sufficiently high density that it will continue to collapse down into a black hole. This is a tricky thing to do, because matter tends to resist collapse, so you need to have quite a bit of force involved. There are a few possibilities, though. One is to scale up the mechanism that we know works for smaller black holes, and start with a larger star.

 This artist’s impression depicts a Sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the Sun, in the centre of a distant galaxy. Its large mass bends the light from stars and gas behind it. Despite being way more massive than the star, the supermassive black hole has an event horizon which is only 200 times larger than the size of the star. Image credit: ESO, ESA/Hubble, M. Kornmesser

This artist’s impression depicts a Sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the Sun, in the centre of a distant galaxy. Its large mass bends the light from stars and gas behind it. Despite being way more massive than the star, the supermassive black hole has an event horizon which is only 200 times larger than the size of the star. Image credit: ESO, ESA/Hubble, M. Kornmesser

On the scale of a supermassive black hole, this pathway is still starting pretty small.  We even have the advantage of starting with the stars in the very earliest Universe, which are thought to likely be hundreds of times more massive than the stars we see near us now. When those larger stars explode, they should leave behind a black hole, which, while larger than the black holes we typically see from stellar explosions nowadays, would still need to grow considerably to reach the size of a supermassive black hole. You’d have to do some combination of feeding that black hole a lot of gas, or merging it with other black holes. But black holes are terrible at gathering gas efficiently into itself in order to grow in mass, and the mergers between black holes are also thought to take quite a long time, though they do happen if you leave them long enough.

Another option is to start large. How do you do that? Well, you could possibly build a tremendously large star, and let it collapse at the end of its life. This collapsing star would have to be tens of thousands times larger than our own Sun (and considerably larger than your standard early-universe star), but that would allow for a black hole many thousands of times more massive than our Sun to form when the star inevitably explodes at the end of its short lifetime. From that larger starting point, you would still need to grow a lot, over time, but if you start large you’d need less building. Going from 10,000 times larger than the Sun to the 1,000,000 times larger than the sun is much, much easier than going from 100 times larger than the Sun to 1,000,000 times larger than the Sun.

These large black holes are probably built through a combination of these possibilities, and potentially some other possibilities we haven’t yet constructed. These questions are part of why we built LIGO, and have plans to build an even more sensitive machine in LISA - those devices will allow us to figure out how common it is for black holes to merge together, and that can help us figure out what the population of black holes looks like in the first place. After all, LIGO has been full of surprises already!

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Looking Into Space, When Do We Start Looking Into The Past?

While observing an astronomical event at a far away distance from Earth, can we consider the events captured by our strongest telescope happening at an earlier time (past event) being captured by the devices (due to the large distance from Earth) or nearly real-time event (with time in reference to that on Earth)?
Sunrise.jpg

Originally posted on Forbes!

It depends very much on how far away you’re looking! Most things out there could be considered “a far away distance”, even when we’re dealing with objects within our own solar system, but the times involved to travel between Mars and the Earth are much closer to nearly real-time than they are if you start venturing further afield.

Fundamentally, information can only travel through the Universe at the speed of light, and the larger your distances get, the longer it takes light to cross those distances. For anything happening on the Earth, this is not usually much of an impediment, because the distances involved in circling the Earth are not so great. To get from the surface of the Earth to the ISS (a distance of 408,000 meters), light, which travels at 299,792,458 meters every second, is only going to spend about a thousandth of a second (0.0013 seconds) in transit. Events on the ISS can therefore be considered pretty much real time, even though there is a measurable communications lag.

If you go further afield, but still within our solar system, light takes about 1.25 seconds to get to the Moon (so a two and a half second round trip), eight minutes to get from the Sun to the Earth, or about twelve and a half minutes to get to Mars. This all starts to build up to a more considerable time delay, but these are manageable delays - if I got an email response from someone I was writing to in less than 24 minutes I’d think that was pretty rapid.

 Hurtling through space at 31,000 miles per hour in this artist's rendering, the New Horizons spacecraft began 21 and a half hours of radio silence as it prepared to collect data for the flyby of Pluto. Image Credit: NASA/APL/SwRI

Hurtling through space at 31,000 miles per hour in this artist's rendering, the New Horizons spacecraft began 21 and a half hours of radio silence as it prepared to collect data for the flyby of Pluto. Image Credit: NASA/APL/SwRI

Once you try talking to the outer solar system, the time delays get a little more significant. The light travel delay to New Horizons when it was swinging past Pluto was about four and a half hours, so to ping New Horizons and hear back instantaneously from the craft, you’d be waiting about nine hours. Somewhere around this kind of time delay, we might start to classify things as happening “in the past”, but this is still a time delay on functional human timescales. Nine hour delays are sending an email to someone and hearing back in the morning. Not so convenient, especially if something complicated is happening in that time, but also not the worst.

It’s when we start looking beyond our solar system and into the Milky Way as a whole, or towards other galaxies that the time delay, which has just been scaling up with the distances involved, gets a little more outrageous. To get information from the center of our own galaxy out to Earth, you have to wait over 26 thousand years. That is no longer a length of time I can wait for an email reply. Information that reaches the Earth from the center of our galaxy is as up to date as it can be, but it’s reporting on changes that happened 26,000 years prior. The changes we see, therefore, are happening at whatever speed we see them happening, but with a time-lag. If we teleported there, it’d be old news.

 This image, captured by the NASA/ESA Hubble Space Telescope, shows what happens when two galaxies become one. The twisted cosmic knot seen here is NGC 2623 — or Arp 243 — and is located about 250 million light-years away in the constellation of Cancer (The Crab). Image credit: ESA/Hubble & NASA

This image, captured by the NASA/ESA Hubble Space Telescope, shows what happens when two galaxies become one. The twisted cosmic knot seen here is NGC 2623 — or Arp 243 — and is located about 250 million light-years away in the constellation of Cancer (The Crab). Image credit: ESA/Hubble & NASA

You can imagine that the further out we go, the bigger this problem gets. So, scrolling outwards, the next big thing is Andromeda, which is so far from us that light has been stretching towards us from those stars for 2.5 million years. I think by most standards, this would be considered observing the past, and yet it’s the closest (and therefore informationally least out of date) galaxy we can look at! Most of the rest of the galaxies in the Universe are much further away, and therefore any changes that happen within them are going to be reported to us by our cosmic messenger in light many millions or billions of years later. The one above is 100 times further away than Andromeda, so news from that galaxy will take 100 times longer to reach us.

Where exactly you feel you should put the boundary between “pretty close to real-time” and “definitely looking at the past” is a bit of an arbitrary, fuzzy boundary. If you want to use “how long would you wait for an email reply” as your metric (as I have here), then your boundary is somewhere within the confines of the solar system. But no matter what you want to put down, there comes a point where we are definitely looking into the past, and certainly by the time we’re looking at other galaxies, we’ve reached it.

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