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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Why Are Earth And Venus Called Twins?

Why are Earth and Venus called twins?
This global view of the surface of Venus is centered at 180 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Image credit: NASA/JPL

This global view of the surface of Venus is centered at 180 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Image credit: NASA/JPL

Originally posted on Forbes!

The Earth and Venus do often get called planetary twins, and this is largely because they are very close to being the same mass. Both the Earth and Venus are rocky planets, which means that they're effectively the same density (which can not be said of the Earth and, say, Neptune), and so they are also very nearly the same physical size. They also both have significant atmospheres surrounding their surfaces. However, their evolutionary pathways since the time of the early solar system have taken both planets down dramatically different tracks, in spite of all their similarities.

The Earth clocks in at a very respectable 5.97 x 10^24 kilograms of mass. Venus sits at 81.5% of this, at 4.867 × 10^24 kg. Venus' radius is only a few hundred kilometers smaller than the Earth's - ~6050 km instead of ~6370, a difference of only about 5%. If we were only looking at their size, rock type, and mass, you might expect that Venus and Earth should be pretty similar, and this is precisely where the “twin” styling comes from.

Computer generated surface view of Eistla Regio (from the northeast). Vertical scale has been exaggerated by a factor of 22.5. Image credit: NASA

Computer generated surface view of Eistla Regio (from the northeast). Vertical scale has been exaggerated by a factor of 22.5. Image credit: NASA

It doesn’t take much further examination to reveal that Venus is a very different place to our home, and much stranger than we originally thought. Unlike most of the other planets in the solar system, which rotate counterclockwise (as viewed looking down from the North Pole), Venus rotates clockwise. (You could equally fairly say that Venus is simply upside down.) It also does so extremely slowly -- Venus is the slowest rotator in the entire solar system. Each ‘day’ lasts for approximately 117 Earth days - in other words, the Earth will have spun 117 times in the time it took Venus to rotate once around its own axis. Considering that Venus’ year is shorter than the Earth’s (it is closer to the Sun, after all), Venus’ year takes about two Venusian days.

This picture of Venus was taken by the Galileo spacecraft's Solid State Imaging System on February 14, 1990, at a range of almost 1.7 million miles from the planet. A highpass spatial filter has been applied in order to emphasize the smaller scale cloud features, and the rendition has been colorized to a bluish hue in order to emphasize the subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. Image credit: NASA/JPL

This picture of Venus was taken by the Galileo spacecraft's Solid State Imaging System on February 14, 1990, at a range of almost 1.7 million miles from the planet. A highpass spatial filter has been applied in order to emphasize the smaller scale cloud features, and the rendition has been colorized to a bluish hue in order to emphasize the subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. Image credit: NASA/JPL

Venus is shrouded with a thick, dense atmosphere, far thicker than our own. On Earth, our atmosphere is thick enough to produce a significant amount of pressure on the surface, but our planet is not totally cloud-covered. Our Earth-monitoring satellites are regularly able to see the ground from space, without the interference of the clouds. There’s no such break in the clouds on Venus. Venus is permanently clouded over, and its atmosphere is so thick that the surface pressure on Venus is 92 times the pressure here on Earth. An unshielded human would fare very badly in this environment.

The atmosphere of Venus is a Level 99 Expert at trapping heat. It’s almost entirely carbon dioxide, which, as we know from our own planet, is a greenhouse gas. With such a high concentration of heat-trapping gas in the atmosphere, the surface temperature of Venus sits at a temperature hotter than 850 degrees Fahrenheit.

Color version of the left half of a Venera 13 image of the surface of Venus. Image credit: NASA

Color version of the left half of a Venera 13 image of the surface of Venus. Image credit: NASA

The crushing pressure and searing heat makes Venus a particularly difficult place to explore. The electronics in most robotic missions generally do not handle these kinds of temperatures very well; to function at all, they need to be reinforced against the pressure. This is the space equivalent of sending your delicate machinery to about 900 meters below the ocean, but where the ocean is 50% of the way to being as hot as lava. We’ve sent relatively few landers to Venus, and the most recent of them (Vega 2) dates back to 1985, operating for just under an hour before succumbing to the heat.

A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground to the base of the mountain, which measures 248 miles across by 0.9 miles high. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory. Image credit: NASA/JPL

A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground to the base of the mountain, which measures 248 miles across by 0.9 miles high. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory. Image credit: NASA/JPL

There is another similarity between the Earth and Venus, though not one that makes Venus a more hospitable place to go visit: both planets have volcanoes. Because Venus is so hot and the pressures are so great, the volcanoes on Venus’ surface aren’t quite as vertically imposing as they can be on Earth. These Venusian volcanoes are much larger, in that they cover a much greater area, but they're extremely flat. Sapas Mons, in the image above, covers 250 miles from edge to edge, but only rises to 4752 feet. (The image above has had its vertical scale exaggerated by a factor of twenty two). This volcano covers roughly the same area as the entire state of New York. Venusian volcanoes dwarf most of their Earthly counterparts in area. I'm biased, but I think you'll agree that Earth has it grandly outdone for scenery.

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Does The Earth's Magnetic Field Go Past The ISS?

Does the Earth’s magnetosphere encompass the ISS and does it offer the same protection as it does our atmosphere and planet?
A profile view of the magnetic field and density data. Image Credit: NASA's Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

A profile view of the magnetic field and density data. Image Credit: NASA's Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

Originally posted on Forbes!

The International Space Station, or ISS, orbits our planet once every 90 minutes at the lofty height of 400 kilometers (about 248 miles) above the surface of our planet. This altitude puts it pretty well above the vast majority of the atmosphere, but it doesn’t place it outside the reaches of the magnetic field which surrounds our Earth.

The International Space Station, in orbit around Earth. Image credit: Science@NASA and NASA’s Goddard Space Flight Center, International Space Station image courtesy of NASA

The International Space Station, in orbit around Earth. Image credit: Science@NASA and NASA’s Goddard Space Flight Center, International Space Station image courtesy of NASA

The magnetic field of our planet — otherwise known as the magnetosphere — extends out to about 65,000 kilometers (40,000 mi) above the surface of the planet. However, that “about” part is pretty critical — the magnetosphere isn’t a fixed boundary, which always remains at exactly 40,000 miles from the surface. This surface is a little more flexible, and if you’ve ever held two opposing magnet ends against each other, you’ve felt this exact flexibility. The resistance between two magnets isn’t a wall, where suddenly you can’t move them any closer to each other. The pressure there is a little more like pressing on a slightly under-inflated balloon.

In the case of the Earth’s magnetic field, the pressure on our magnetic field comes from the Sun. The Sun is constantly battering everything surrounding it with a solar wind, made up of charged particles. If you’re a planet without a protective magnetic field, this solar wind will slam into your atmosphere, and can destroy it over time. This is roughly what we believe happened to Mars’ atmosphere. The Earth has a very fortunate protective shield, but this constant pressure on the Sun’s side of our planet means that this magnetic protection is pressed back, closer to the planet. That 40,000 mile number I gave is this: the typical, Sun-compressed, Sun-facing side of our magnetosphere. The nighttime side of our planet, facing away from the Sun, has a long magnetic tail drifting out beyond it, extending several times farther out than the Sun-side.

A profile view of the magnetic field and density data during a solar outburst. Image Credit: NASA’s Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

A profile view of the magnetic field and density data during a solar outburst. Image Credit: NASA’s Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

If the Sun has a particularly strong outburst – a coronal mass ejection or any kind of solar flare — the pressure on our magnetic field gets much stronger, but nothing the Sun typically does will press the magnetic field down close enough to the Earth’s atmosphere so that the ISS would exit the magnetic field. Some of our highest orbiting satellites do exit the magnetic field of the Earth, as of course all craft going to other planets must also do. However, these satellites and spacecraft must be constructed to protect their inner workings from the charged particles in the solar wind. Satellites are effectively very elaborate electronics, and electronics do not like being exposed to charged particles. It shorts their circuits.

All this really means that the ISS is in a much safer region of space than it could be – not that it’s totally safe. Our magnetic field is not a perfect blocker of high energy particles, and so things like gamma rays, cosmic rays, and other damaging radiation can still appear in higher quantities than they would if the astronauts were still safely on  the ground. Our atmosphere is pretty good at blocking a lot of these high energy particles, so on the ground you’d never get exposed to them. But the ISS is above the atmosphere, and doesn’t have this extra layer of shielding, so there are radiation monitors on the space station to keep track of how much of a radiation dose they’re getting. If a solar flare is on its way, the astronauts usually have a few days’ warning, and can take shelter in more strongly shielded section of the ISS if they need to. (Not all solar storm are aimed in their direction, and not all storms are strong enough to require this precaution.)

So yes, the ISS is firmly embedded in the Earth’s magnetosphere, making it — for a space based outpost — a relatively safe haven for our astronauts.

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How Fast Are You Moving Through Space?

While standing in my back yard, how fast am I moving through space considering Earth’s rotation, Earth’s orbit around the Sun, the Sun’s orbit around the Galaxy, et cetera?
This image, taken by ESO Photo Ambassador Alexandre Santerne, is more than a little disorientating at first glance! Resembling an optical illusion or an abstract painting, the starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory in Chile. Each circular streak represents an individual star, imaged over a long period of time to capture the motion of the stars across the sky caused by the Earth’s rotation. La Silla is based in the outskirts of Chile’s Atacama Desert at some 2400 metres above sea level, and offers perfect observing conditions for long-exposure shots like this; the site experiences over 300 clear nights a year! The site is host to many of ESO’s telescopes and to national projects run by the ESO Member States. Some of these telescopes can be seen towards the bottom of the image. The ESO 3.6-metre telescope stands tall on the left peak, now home to the world's foremost extrasolar planet hunter: the High Accuracy Radial velocity Planet Searcher (HARPS). Other telescopes at La Silla include the New Technology Telescope, which partly masks the ESO 3.6-metre telescope, Swiss 1.2-metre Leonhard Euler Telescope, ESO 1-metre Schmidt, the silver-domed MPG/ESO 2.2-metre, Danish 1.54-metre, and ESO 1.52-metre telescopes, which are visible here. Taking all these facilities together, La Silla is one of the most scientifically productive ground-based facilities in the world after ESO’s Very Large Telescope (VLT) observatory. With almost 300 refereed publications attributable to the work of the observatory per year, La Silla remains at the forefront of astronomy. Image credit: ESA/ A. Santerne

This image, taken by ESO Photo Ambassador Alexandre Santerne, is more than a little disorientating at first glance! Resembling an optical illusion or an abstract painting, the starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory in Chile. Each circular streak represents an individual star, imaged over a long period of time to capture the motion of the stars across the sky caused by the Earth’s rotation. La Silla is based in the outskirts of Chile’s Atacama Desert at some 2400 metres above sea level, and offers perfect observing conditions for long-exposure shots like this; the site experiences over 300 clear nights a year! The site is host to many of ESO’s telescopes and to national projects run by the ESO Member States. Some of these telescopes can be seen towards the bottom of the image. The ESO 3.6-metre telescope stands tall on the left peak, now home to the world's foremost extrasolar planet hunter: the High Accuracy Radial velocity Planet Searcher (HARPS). Other telescopes at La Silla include the New Technology Telescope, which partly masks the ESO 3.6-metre telescope, Swiss 1.2-metre Leonhard Euler Telescope, ESO 1-metre Schmidt, the silver-domed MPG/ESO 2.2-metre, Danish 1.54-metre, and ESO 1.52-metre telescopes, which are visible here. Taking all these facilities together, La Silla is one of the most scientifically productive ground-based facilities in the world after ESO’s Very Large Telescope (VLT) observatory. With almost 300 refereed publications attributable to the work of the observatory per year, La Silla remains at the forefront of astronomy. Image credit: ESA/A. Santerne

Originally posted at Forbes!

We can certainly figure this one out, with the caveats presented in one of my last articles – all these speeds that we measure will be with respect to something specific, and also arbitrary. So we can find out how fast you’re moving on the Earth’s surface because the Earth is rotating, relative to standing at the North Pole, where you would spin in place. We can figure out how fast you’re moving because the Earth is orbiting the Sun, relative to standing at the Sun. And we can figure out how fast we’re moving around the galaxy, relative to standing at the very center of the galaxy, and spinning that way.

Fair warning: this is going to get complicated. All of the motions I’m telling you about will be constantly changing direction, since everything is on a circular path. For instance, if the sun is setting, your direction of motion is roughly “away from the sun”. If the sun is rising, you’re roughly pointed “towards the sun”, and at midnight and midday, you’re moving sideways, relative to the sun.

On the left; a top down diagram of the Earth, showing the different directions your instantaneous motion would point, depending on your position on the surface. On the right; a side view, showing Earth’s rotational tilt, and the directions your motion points, where horizontal is direction of motion relative to the orbit around the sun. Image credit: Jillian Scudder

On the left; a top down diagram of the Earth, showing the different directions your instantaneous motion would point, depending on your position on the surface. On the right; a side view, showing Earth’s rotational tilt, and the directions your motion points, where horizontal is direction of motion relative to the orbit around the sun. Image credit: Jillian Scudder

So let’s start small. You are standing on a spherical planet (more or less) which rotates once every 24 hours. This is a 360 degree rotation in 24 hours, which works out to 15 degrees of rotation every hour. Cool, but you asked for your motion, which means we need to convert from angles to a velocity. If you draw a circle at a fixed rate (say you can draw an inch every second), the length of time you’re going to take to draw that circle depends on how big a circle you’re drawing. With the rotation of the Earth, you have almost the exact opposite problem. We know how much time it takes to draw the circle, but we don’t know how big the circle is, because it depends on where you are on the planet. If your backyard happens to be 3 feet away from the North Pole, then your circle is only 6 feet across, and you have 24 hours to make it around. Unsurprisingly, your speed here is really slow. You’re only going 10 inches per hour.

If your backyard is on the equator, your circle is much bigger – it’s the whole Earth across. The Earth is just over 7900 miles from surface to surface, if you tunnel through the middle, which means you have a lot farther to go in the same 24 hours. Your speed clocks in around 1,036 miles per hour, relative to the “motionless” North Pole.

We can do this again, scaled up for the Earth’s motion around the sun – it’s a little bit simpler because we know how big the circle drawn by the earth is – 1 astronomical unit, or just under 93 million miles lie between us and the Sun, which means we have a 584 million mile journey to make it through before the year is out. You’ll note that 584 million miles is a larger number of million miles than there are days in the year – and indeed this works out to needing to travel 1.6 million miles per day along our circular path. If we break this into units of miles per hour again, we get 66,658 mph.

To combine your motion around the Earth with your motion around the sun, you should add the two together, taking into consideration the relative directions. If you motion around the Earth happens to be pointed in the direction of the Earth’s motion around the sun, you need to add the two motions together. If you happen to be pointed away from the direction we’re moving as a planet, then you need to subtract the two. If you’re at the North Pole, or very close to it, this addition and subtraction makes no real difference to our 66,600 mph motion along our orbit, since you’re going almost 0 mph in the first place.

Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Image credit: Dennis Nilsson CC BY 3.0

Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Image credit: Dennis Nilsson CC BY 3.0

There’s one more complication in adding these two things together! The direction of your motion because the Earth is spinning (90 degrees from our axis of rotation) is not lined up with the direction of the motion of the Earth around the Sun. Our rotation is tipped sideways by 23 degrees. In that case, even if you’re pointed mostly toward the direction we’re moving around the sun, you’re also pointed up a little (or down a little). It’s like walking in a diagonal line, forward and to the right, but then realising you only cared about moving right. The forward motion you made doesn’t really help with moving to the right.

We can work out how much of our motion was “to the right” in the direction of the motion around the sun, and sum just those two segments together, but we can’t forget about our “up/down” motion either. If you figure out the geometry, our 1,036 mph of motion at the equator is equivalent to going 954 mph to the side, along our direction of motion. Or against our direction of motion – remember that as the planet rotates, our direction of motion is also changing, so depending on the time of day, our rotation around the planet will add an extra 954 mph at most, subtract 954 mph at most, and at the least consequence, make no change whatsoever, all relative to our hypothetical viewer in the Sun who can see us no matter what side of the planet we’re on. So your motion will appear to be somewhere between 65,700 mph and 67,600 mph in the direction of the planet’s motion, plus an up-or-down component somewhere between 0 mph and 400 mph depending on season and time of day.

This detailed annotated artist’s impression shows the structure of the Milky Way, including the location of the spiral arms and other components such as the bulge. This version of the image has been updated to include the most recent mapping of the shape of the central bulge deduced from survey data from ESO’s VISTA telescope at the Paranal Observatory. Image credit: NASA/JPL-Caltech/ESO/R. Hurt

This detailed annotated artist’s impression shows the structure of the Milky Way, including the location of the spiral arms and other components such as the bulge. This version of the image has been updated to include the most recent mapping of the shape of the central bulge deduced from survey data from ESO’s VISTA telescope at the Paranal Observatory. Image credit: NASA/JPL-Caltech/ESO/R. Hurt

It’s the same verse again for calculating the speed of the solar system around the Galaxy’s center, but with an extra round of calculating angles. Our solar system is about 8,300 parsecs from the center of the galaxy – where one parsec is 19,170,000,000,000 miles. (This sort of enormous number gets inconvenient, so for any kind of science, I would prefer to calculate things in parsecs rather than in miles.) With our best measurements of our own speed around the center of the galaxy, we’ve estimated our speed to sit somewhere around 220 kilometers every second, or 492,126 miles per hour. Our last set of numbers once again does not add easily, as the solar system sits at a 60 degree angle relative to the Milky Way galaxy – so if you do the math again, you can work out that we can at most add or subtract 34,156 mph to our sideways speed, getting us to somewhere between 526,282 mph and 457,970 mph. You’ll only hit those extremes once a year, at the right time of day, and it will only last an instant – most of the time we’ll be sitting somewhere much closer to the 492,126 mph average.

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