Here on Earth, we to end to not give our measurements of time much thought. Unless we’re griping about Time Zones, enjoying the extra day of a Leap Year, or contemplating the rationality of Daylight Savings Time, we tend to take it all for granted. But when you consider the fact that increments like a year are entirely relative, dependent on a specific space and place, you begin to see how time really works.
Here on Earth, we consider a year to be 365 days. Unless of course it’s a Leap Year, which takes place every four years (in which it is 366). But the actual definition of a year is the time it takes our planet to complete a single orbit around the Sun. So if you were to put yourself in another frame of reference – say, another planet – a year would work out to something else. Let’s see just how long a year is on the other planets, shall we?
Rare #thundersnow visible from @Space_Station in #blizzard2016! Jan. 23, 2016. Credit: NASA/Scott Kelly/@StationCDRKelly
Rare #thundersnow visible from @Space_Station in #blizzard2016! Jan. 23, 2016. Credit: NASA/Scott Kelly/@StationCDRKelly
NEW JERSEY – NASA astronaut Scott Kelly captured a rare and spectacular display of ‘thundersnow’ from space as Snowzilla’s blast pummeled much of the US East Coast this weekend with two feet or more of paralyzing snow from the nations’ capital to New York City and beyond.
This is Kepler 186f, an exoplanet in the habitable zone around a red dwarf. We've found many planets in their stars' habitable zones where they could potentially have surface water. But it's a fairly crude understanding of true habitability. Image Credit: NASA Ames, SETI Institute, JPL-Caltech, T. Pyle)
In 1950, physicist Enrico Fermi raised a very important question about the Universe and the existence of extraterrestrial life. Given the size and age of the Universe, he stated, and the statistical probability of life emerging in other solar systems, why is it that humanity has not seen any indications of intelligent life in the cosmos? This query, known as the Fermi Paradox, continues to haunt us to this day.
If, indeed, there are billions of star systems in our galaxy, and the conditions needed for life are not so rare, then where are all the aliens? According to a recent paper by researchers at Australian National University’s Research School of Earth Sciences., the answer may be simple: they’re all dead. In what the research teams calls the “Gaian Bottleneck”, the solution to this paradox may be that life is so fragile that most of it simply doesn’t make it.
Professor Stephen Hawking enjoying a lighter moment, and not contemplating the end of humanity. Image credit: Zero G
If you’re thinking of having yourself cryogenically suspended and awakened in some future paradise, you might want to set your alarm clock for no later than 1,000 years from now. According to the BBC, Stephen Hawking will be saying this much in the 2016 Reith Lectures – a series of lectures organized by the BBC that explore the big challenges faced by humanity.
In Hawking’s first lecture, which will be broadcast on February 26th on the BBC, Hawking covers the topic of black holes, whether or not they have hair, and other concepts about these baffling objects.
But at the end of the lecture, he responded to audience questions about humanity’s capacity for self destruction. Hawking said that 1,000 years might be all we have until we meet our demise at the hands of our own scientific and technological advances.
As we have become increasingly advanced both scientifically and technologically, Hawking says, we will be creating “new ways that things can go wrong.” Hawking mentioned nuclear war, global warming, and genetically engineered viruses as things that could cause our extinction.
Nuclear War
Through the Cold War, annihilation at the hands of our own nuclear weapons was a real danger. The threat of a nuclear launch in response to a real or perceived threat was real. The resulting retaliation and counter-retaliation was a risk faced by everyone on the planet. And the two superpowers had enough warheads between them to potentially wipe out life on Earth.
One nuclear explosion can ruin your whole day. Image: Andrew Kuznetsov, CC by 2.0
The USA and the USSR have reduced their stockpiles of nuclear weapons in recent decades, but there are still enough warheads around to wipe us out. The possibility of a rogue state like North Korea setting off a nuclear confrontation is still very real. By the time Hawking’s 1,000 year time-frame has passed, we’ll either have solved this problem, or we won’t be here.
Global Warming
Earth is getting warmer, and though the Earth has warmed and cooled many times in its history, this time we only have ourselves to blame. We’ve been inadvertently enriching our atmosphere with carbon since the Industrial Revolution. All that carbon is creating a nice insulating layer around Earth, as it traps heat that would normally radiate into space. If we reach some of the “tipping points” that scientists talk about, like the melting of permafrost and the subsequent release of methane, we could be in real trouble.
Global Mean Surface Temperature. Image: NASA, Goddard Institute for Space Studies
Different climate engineering schemes have been thought up to counteract global warming, like seeding the upper atmosphere with reflective molecules, and having fleets of ships around the equator spraying sea mist into the air to partially block out the sun. Or even extracting carbon from the atmosphere. But how realistic or effective those counter-measures might be is not clear.
Genetically Engineered Viruses
As a weapon, a virus can be cheap and effective. There’ve been programs in the past to develop biological weapons. The temptation to use genetic science to create extremely deadly viruses may prove too great.
Smallpox and Viral Hemorrhagic Fevers have been weaponized, and as our genetic manipulation abilities grow, it’s possible, or even likely, that somebody somewhere will attempt develop even more dangerous viral weapons. They may be doing it right now.
Hawking never mentioned AI in his talk, but it fits in with the discussion. As our machines get smarter and smarter, will they deduce that the only chance for survival is to remove or reduce the human population? Who knows. But Hawking himself, as well as other thinkers, have been warning us that there may be a catastrophic downside to our achievements in AI.
A Google driverless car: Looks harmless, doesn’t it? Image: Michael Shick http://creativecommons.org/licenses/by-sa/4.0
We may love the idea of driverless cars, and computer assistants like SIRI. But as numerous science fiction stories have warned us (Skynet in the Terminator series being my favorite,) it may be a small step from very helpful AI that protects us and makes our lives easier, to AI that decides existence would be a whole lot better without us pesky humans around.
The Technological Singularity is the point at which artificially intelligent systems “wake up” and become—more or less—conscious. These AI machines would start to improve themselves recursively, or build better and smarter machines. At this point, they would be a serious danger to humanity.
Drones are super popular right now. They flew off the shelves at Christmas, and they’re great toys. But once we start seeing drones with primitive but effective AI, patrolling the property of the wealthy, it’ll be time to start getting nervous.
Extinction May Have To Wait
As our scientific and technological prowess grows, we’ll definitely face new threats, just like Hawking says. But, that same progress may also protect us, or make us more resilient. Hawking says, “We are not going to stop making progress, or reverse it, so we have to recognise the dangers and control them. I’m an optimist, and I believe we can.” So do we.
Maybe you’ll be able to hit the snooze button after all.
Planets and other objects in our Solar System. Credit: NASA.
Here on Earth, we tend to take time for granted, never suspected that the increments with which we measure it are actually quite relative. The ways in which we measure our days and years, for example, are actually the result of our planet’s distance from the Sun, the time it takes to orbit, and the time it takes to rotate on its axis. The same is true for the other planets in our Solar System.
While we Earthlings count on a day being about 24 hours from sunup to sunup, the length of a single day on another planet is quite different. In some cases, they are very short, while in others, they can last longer than years – sometimes considerably! Let’s go over how time works on other planets and see just how long their days can be, shall we?
A Day On Mercury:
Mercury is the closest planet to our Sun, ranging from 46,001,200 km at perihelion (closest to the Sun) to 69,816,900 km at aphelion (farthest). Since it takes 58.646 Earth days for Mercury to rotate once on its axis – aka. its sidereal rotation period – this means that it takes just over 58 Earth days for Mercury to experience a single day.
However, this is not to say that Mercury experiences two sunrises in just over 58 days. Due to its proximity to the Sun and rapid speed with which it circles it, it takes the equivalent of 175.97 Earth days for the Sun to reappear in the same place in the sky. Hence, while the planet rotates once every 58 Earth days, it is roughly 176 days from one sunrise to the next on Mercury.
Images of Mercury’s northern polar region, provided by MESSENGER. Credit: NASA/JPL
What’s more, it only takes Mercury 87.969 Earth days to complete a single orbit of the Sun (aka. its orbital period). This means a year on Mercury is the equivalent of about 88 Earth days, which in turn means that a single Mercurian (or Hermian) year lasts just half as long as a Mercurian day.
What’s more, Mercury’s northern polar regions are constantly in the shade. This is due to it’s axis being tilted at a mere 0.034° (compared to Earth’s 23.4°), which means that it does not experience extreme seasonal variations where days and nights can last for months depending on the season. On the poles of Mercury, it is always dark and shady. So you could say the poles are in a constant state of twilight.
A Day On Venus:
Also known as “Earth’s Twin”, Venus is the second closest planet to our Sun – ranging from 107,477,000 km at perihelion to 108,939,000 km at aphelion. Unfortunately, Venus is also the slowest moving planet, a fact which is made evident by looking at its poles. Whereas every other planet in the Solar System has experienced flattening at their poles due to the speed of their spin, Venus has experienced no such flattening.
Venus has a rotational velocity of just 6.5 km/h (4.0 mph) – compared to Earth’s rational velocity of 1,670 km/h (1,040 mph) – which leads to a sidereal rotation period of 243.025 days. Technically, it is -243.025 days, since Venus’ rotation is retrograde. This means that Venus rotates in the direction opposite to its orbital path around the Sun.
The planet Venus, as imagined by the Magellan 10 mission. Credit: NASA/JPL
So if you were above Venus’ north pole and watched it circle around the Sun, you would see it is moving clockwise, whereas its rotation is counter-clockwise. Nevertheless, this still means that Venus takes over 243 Earth days to rotate once on its axis. However, much like Mercury, Venus’ orbital speed and slow rotation means that a single solar day – the time it takes the Sun to return to the same place in the sky – lasts about 117 days.
So while a single Venusian (or Cytherean) year works out to 224.701 Earth days, it experiences less than two full sunrises and sunsets in that time. In fact, a single Venusian/Cytherean year lasts as long as 1.92 Venusian/Cytherean days. Good thing Venus has other things in common With Earth, because it is sure isn’t its diurnal cycle!
A Day On Earth:
When we think of a day on Earth, we tend to think of it as a simple 24 hour interval. In truth, it takes the Earth exactly 23 hours 56 minutes and 4.1 seconds to rotate once on its axis. Meanwhile, on average, a solar day on Earth is 24 hours long, which means it takes that amount of time for the Sun to appear in the same place in the sky. Between these two values, we say a single day and night cycle lasts an even 24.
At the same time, there are variations in the length of a single day on the planet based on seasonal cycles. Due to Earth’s axial tilt, the amount of sunlight experienced in certain hemispheres will vary. The most extreme case of this occurs at the poles, where day and night can last for days or months depending on the season.
At the North and South Poles during the winter, a single night can last up to six months, which is known as a “polar night”. During the summer, the poles will experience what is called a “midnight sun”, where a day lasts a full 24 hours. So really, days are not as simple as we like to imagine. But compared to the other planets in the Solar System, time management is still easier here on Earth.
A Day On Mars:
In many respects, Mars can also be called “Earth’s Twin”. In addition to having polar ice caps, seasonal variations , and water (albeit frozen) on its surface, a day on Mars is pretty close to what a day on Earth is. Essentially, Mars takes 24 hours 37 minutes and 22 seconds to complete a single rotation on its axis. This means that a day on Mars is equivalent to 1.025957 days.
The seasonal cycles on Mars, which are due to it having an axial tilt similar to Earth’s (25.19° compared to Earth’s 23.4°), are more similar to those we experience on Earth than on any other planet. As a result, Martian days experience similar variations, with the Sun rising sooner and setting later in the summer and then experiencing the reverse in the winter.
However, seasonal variations last twice as long on Mars, thanks to Mars’ being at a greater distance from the Sun. This leads to the Martian year being about two Earth years long – 686.971 Earth days to be exact, which works out to 668.5991 Martian days (or Sols). As a result, longer days and longer nights can be expected last much longer on the Red Planet. Something for future colonists to consider!
Sunrise at Gale Crater on Mars. Gale is at center top with the mound in the middle, called Mt. Sharp (Aeolis Mons.)
A Day On Jupiter:
Given the fact that it is the largest planet in the Solar System, one would expect that a day on Jupiter would last a long time. But as it turns out, a Jovian day is officially only 9 hours, 55 minutes and 30 seconds long, which means a single day is just over a third the length of an Earth day. This is due to the gas giant having a very rapid rotational speed, which is 12.6 km/s (45,300 km/h, or 28148.115 mph) at the equator. This rapid rotational speed is also one of the reasons the planet has such violent storms.
Note the use of the word officially. Since Jupiter is not a solid body, its upper atmosphere undergoes a different rate of rotation compared to its equator. Basically, the rotation of Jupiter’s polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere. Because of this, astronomers use three systems as frames of reference.
System I applies from the latitudes 10° N to 10° S, where its rotational period is the planet’s shortest, at 9 hours, 50 minutes, and 30 seconds. System II applies at all latitudes north and south of these; its period is 9 hours, 55 minutes, and 40.6 seconds. System III corresponds to the rotation of the planet’s magnetosphere, and it’s period is used by the IAU and IAG to define Jupiter’s official rotation (i.e. 9 hours 44 minutes and 30 seconds)
Jupiter and Io capturing the Sun. Image Credit: NASA/JPL
So if you could, theoretically, stand on the cloud tops of Jupiter (or possibly on a floating platform in geosynchronous orbit), you would witness the sun rising an setting in the space of less than 10 hours from any latitude. And in the space of a single Jovian year, the sun would rise and set a total of about 10,476 times.
A Day On Saturn:
Saturn’s situation is very similar to that of Jupiter’s. Despite its massive size, the planet has an estimated rotational velocity of 9.87 km/s (35,500 km/h, or 22058.677 mph). As such Saturn takes about 10 hours and 33 minutes to complete a single sidereal rotation, making a single day on Saturn less than half of what it is here on Earth. Here too, this rapid movement of the atmosphere leads to some super storms, not to mention the hexagonal pattern around the planet’s north pole and a vortex storm around its south pole.
And, also like Jupiter, Saturn takes its time orbiting the Sun. With an orbital period that is the equivalent of 10,759.22 Earth days (or 29.4571 Earth years), a single Saturnian (or Cronian) year lasts roughly 24,491 Saturnian days. However, like Jupiter, Saturn’s atmosphere rotates at different speed depending on latitude, which requires that astronomers use three systems with different frames of reference.
System I encompasses the Equatorial Zone, the South Equatorial Belt and the North Equatorial Belt, and has a period of 10 hours and 14 minutes. System II covers all other Saturnian latitudes, excluding the north and south poles, and have been assigned a rotation period of 10 hr 38 min 25.4 sec. System III uses radio emissions to measure Saturn’s internal rotation rate, which yielded a rotation period of 10 hr 39 min 22.4 sec.
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
Using these various systems, scientists have obtained different data from Saturn over the years. For instance, data obtained during the 1980’s by the Voyager 1 and 2 missions indicated that a day on Saturn was 10 hours 39 minutes and 24 seconds long. In 2004, data provided by the Cassini-Huygens space probe measured the planet’s gravitational field, which yielded an estimate of 10 hours, 45 minutes, and 45 seconds (± 36 sec).
In 2007, this was revised by researches at the Department of Earth, Planetary, and Space Sciences, UCLA, which resulted in the current estimate of 10 hours and 33 minutes. Much like with Jupiter, the problem of obtaining accurate measurements arises from the fact that, as a gas giant, parts of Saturn rotate faster than others.
A Day On Uranus:
When we come to Uranus, the question of how long a day is becomes a bit complicated. One the one hand, the planet has a sidereal rotation period of 17 hours 14 minutes and 24 seconds, which is the equivalent of 0.71833 Earth days. So you could say a day on Uranus lasts almost as long as a day on Earth. It would be true, were it not for the extreme axial tilt this gas/ice giant has going on.
With an axial tilt of 97.77°, Uranus essentially orbits the Sun on its side. This means that either its north or south pole is pointed almost directly at the Sun at different times in its orbital period. When one pole is going through “summer” on Uranus, it will experience 42 years of continuous sunlight. When that same pole is pointed away from the Sun (i.e. a Uranian “winter”), it will experience 42 years of continuous darkness.
Uranus as seen by NASA’s Voyager 2. Credit: NASA/JPL
Hence, you might say that a single day – from one sunrise to the next – lasts a full 84 years on Uranus! In other words, a single Uranian day is the same amount of time as a single Uranian year (84.0205 Earth years).
In addition, as with the other gas/ice giants, Uranus rotates faster at certain latitudes. Ergo, while the planet’s rotation is 17 hours and 14.5 minutes at the equator, at about 60° south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.
A Day On Neptune:
Last, but not least, we have Neptune. Here too, measuring a single day is somewhat complicated. For instance, Neptune’s sidereal rotation period is roughly 16 hours, 6 minutes and 36 seconds (the equivalent of 0.6713 Earth days). But due to it being a gas/ice giant, the poles of the planet rotate faster than the equator.
Whereas the planet’s magnetic field has a rotational speed of 16.1 hours, the wide equatorial zone rotates with a period of about 18 hour. Meanwhile, the polar regions rotate the fastest, at a period of 12 hours. This differential rotation is the most pronounced of any planet in the Solar System, and it results in strong latitudinal wind shear.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
In addition, the planet’s axial tilt of 28.32° results in seasonal variations that are similar to those on Earth and Mars. The long orbital period of Neptune means that the seasons last for forty Earth years. But because its axial tilt is comparable to Earth’s, the variation in the length of its day over the course of its long year is not any more extreme.
As you can see from this little rundown of the different planets in our Solar System, what constitutes a day depends entirely on your frame of reference. In addition to it varying depending on the planet in question, you also have to take into account seasonal cycles and where on the planet the measurements are being taken from.
As Einstein summarized, time is relative to the observer. Based on your inertial reference frame, its passage will differ. And when you are standing on a planet other than Earth, your concept of day and night, which is set to Earth time (and a specific time zone) is likely to get pretty confused!
The SpaceX Falcon 9 rocket is seen as it launches from Vandenberg Air Force Base Space Launch Complex 4 East with the Jason-3 spacecraft onboard, , Sunday, Jan. 17, 2016, Vandenberg Air Force Base, California. Jason-3, an international mission led by the National Oceanic and Atmospheric Administration (NOAA), will help continue U.S.-European satellite measurements of global ocean height changes. Photo Credit: (NASA/Bill Ingalls)
The SpaceX Falcon 9 rocket is seen as it launches from Vandenberg Air Force Base Space Launch Complex 4 East with the Jason-3 spacecraft onboard, Sunday, Jan. 17, 2016, Vandenberg Air Force Base, California. Jason-3, an international mission led by the National Oceanic and Atmospheric Administration (NOAA), will help continue U.S.-European satellite measurements of global ocean height changes. Photo Credit: (NASA/Bill Ingalls)
What would Earth look like if it had a ring system like Saturn's. Credit: Kevin Gill/Flickr
Saturn’s Rings are amazing to behold. Since they were first observed by Galileo in 1610, they have been the subject of endless scientific interest and popular fascination. Composed of billions of particles of dust and ice, these rings span a distance of about 282,000 km (175,000 miles) – which is three quarters of the distance between the Earth and its Moon – and hold roughly 30 quintillion kilograms (that’s 3.0. x 1018 kg) worth of matter.
All of the Solar System’s gas giants, from Jupiter to Neptune, have their own ring system – albeit less visible and picturesque ones. Sadly, none of the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) have such a system. But just what would it look like if Earth did? Putting aside the physical requirements that it would take for a ring system to exist, what would it be like to look up from Earth and see beautiful rings reaching overhead?
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)
During the many thousand years that human beings have been looking up at the stars, our concept of what the Universe looks like has changed dramatically. At one time, the magi and sages of the world believed that the Universe consisted of a flat Earth (or a square one, a zigarrut, etc.) surrounded by the Sun, the Moon, and the stars. Over time, ancient astronomers became aware that some stars did not move like the rest, and began to understand that these too were planets.
In time, we also began to understand that the Earth was indeed round, and came up with rationalized explanations for the behavior of other celestial bodies. And by classical antiquity, scientists had formulated ideas on how the motion of the planets occurred, and how all the heavenly orbs fit together. This gave rise to the Geocentric model of the universe, a now-defunct model that explained how the Sun, Moon, and firmament circled around our planet.
The lunar module above the Moon during the Apollo 11 mission (NASA)
Back in 2008, Richard Branson outlined his vision for Virgin Galactic’s future. Once tourists are taken into Earth orbit, it seems possible that space hotels could be developed for longer stop-overs in space. He then went on to mention that short “sight-seeing” tours to the Moon could be started from these ultimate hotels. If we are to make travel to the Moon routine enough to send tourists there, the trip would need to be as short as possible.
So how long is the commute from the Earth to the Moon anyway? Human beings and machines have made that trip on several occasions. And while some took a very long time, others were astonishingly fast. Let’s review the various missions and methods, and see which offers the most efficient and least time-consuming means of transit.
