Mercury is one of the most unusual planets in our Solar System, at least by the standards of us privileged Earthlings. Despite being the closest planet to our Sun, it is not the hottest (that honor goes to Venus). And because of its virtually non-existence atmosphere and slow rotation, temperatures on its surface range from being extremely hot to extremely cold.
Equally unusual is the diurnal cycle on Mercury – i.e. the cycle of day and night. A single year lasts only 88 days on Mercury, but thanks again to its slow rotation, a day lasts twice as long! That means that if you could stand on the surface of Mercury, it would take a staggering 176 Earth days for the Sun to rise, set and rise again to the same place in the sky just once!
Distance and Orbital Period:
Mercury is the closest planet to our Sun, but it also has the most eccentric orbit (0.2056) of any of the Solar Planets. This means that while its average distance (semi-major axis) from the Sun is 57,909,050 km (35,983,015 mi) or 0.387 AUs, this ranges considerably – from 46,001,200 km (2,8583,820 mi) at perihelion (closet) to 69,816,900 km (43,382,210 mi) at aphelion (farthest).
Because of this proximity, Mercury has a rapid orbital period, which varies depending on where it is in its orbit. Naturally, it moves fastest when it is at its closest to the Sun, and slowest when it is farthest. On average, its orbital velocity is 47.362 km/s (29.43 mi/s), which means it takes only 88 days to complete a single orbit of the Sun.
Astronomers used to suspect that Mercury was tidally locked to the Sun, meaning that it always showed the same face to the Sun – similar to how the Moon is tidally locked to the Earth. But radar-Doppler measurements obtained in 1965 demonstrated that Mercury is actually rotating very slowly compared to the Sun.
Sidereal vs. Solar Day:
Based on data obtained by these radar measurements, Mercury is now known to be in 3:2 orbital resonance with the Sun. This means that the planet completes three rotations on its axis for every two orbits it makes around the Sun. At it’s current rotational velocity – 3.026 m/s, or 10.892 km/h (6.77 mph) – it takes Mercury 58.646 days to complete a single rotation on its axis.
While this might lead some to conclude that a single day on Mercury is about 58 Earth days – thus making the length of a day and year correspond to the same 3:2 ratio – this would be inaccurate. Due to its rapid orbital velocity and slow sidereal rotation, a Solar Day on Mercury (the time it takes for the Sun to return to the same place in the sky) is actually 176 days.
In that respect, the ratio of days to years on Mercury is actually 1:2. The only places that are exempt to this day and night cycle are the polar regions. The cratered northern polar region, for example, exists in a state of perpetual shadow. Temperatures in these craters are also cool enough that significant concentrations of water ice can exist in stable form.
For over 20 years, scientists believed that radar-bright images from Mercury’s northern polar regions might indicate the presence of water ice there. In November of 2012, NASA’s MESSENGER probe examined the northern polar region using its neutron spectrometer and laser altimeter and confirmed the presence of both water ice and organic molecules.
Yes, as if Mercury weren’t strange enough, it turns out that a single day on Mercury lasts as long as two years! Just another oddity for a planet that likes to keep things really hot, really cold, and is really eccentric.
Mercury is a planet of extremes. As the closest planet to our Sun, it experiences extremely high surface temperatures. But since it has virtually no atmosphere to speak of, and rotates very slowly on its axis, it gravitates between extremes of hot and cold. It also means that it’s Sun-facing side experiences prolonged periods of day while its dark side experiences extremely long periods of night.
It’s proximity to the Sun also means that it orbits the planet quite rapidly. To break it down, Mercury takes roughly 88 Earth days to complete a single orbit around the Sun. Between this rapid orbital period and its slow rotational period, a single year on Mercury is actually shorter than a single day!
Mercury orbits the Sun at a distance of 57,909,050 km (35,983,015 mi), which works out to o.387 AU – or slightly more than one-third the distance between the Sun and the Earth. It’s orbit is also highly eccentric, ranging from a distance of 46 million km/28.58 million mi at its closest (perihelion) to 70 million km/43.49 million mi at its most distant (aphelion).
Like all the planets, Mercury moves fastest when it is at its closest point to the Sun, and slowest when it is at its farthest. However, it’s proximity to the Sun means that its average orbital velocity is a speedy 47.362 kilometers a second or 29.429 miles per second – approximately 170,500 km/h; 105,945 mph.
At this rate, it takes Mercury 87.969 days, or the equivalent of 0.24 Earth years, to complete a single orbit of the Sun. Thus, it can be said that a year on Mercury lasts almost as long as 3 months here on Earth.
Sidereal and Solar Day:
Astronomers used to think that Mercury was tidally locked to the Sun, where its rotational period matched its orbital period. This would mean that the same side it always pointed towards the Sun, thus ensuring that one side was perennially sunny (and extremely hot) while the other experienced constant night (and freezing cold).
However, improved observations and studies of the planet have led scientists to conclude that in fact, the planet has a slow rotational period of 58.646 days. Compared to its orbital period of 88 days, this means that Mercury has a spin-orbit resonance of 3:2, which means that the planet makes three completes rotations on its axis for every two orbits it makes around the Sun.
Another consequences of its spin-orbit resonance is that there is a significance difference between the time it takes the planet to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes a 176 days for the Sun to rise, set, and return to the same place in the sky. This means, effectively, that a single day on Mercury lasts as long as two years!
Yes, Mercury is a pretty extreme place. Not only do temperatures on its surface range from molten hot to freezing cold, but a single day lasts as long as six months here on Earth. Add to that the fact that it has virtually no atmosphere, and is exposed to extreme amounts of radiation, and you can begin to understand why life cannot exist there.
Venus is often referred to as “Earth’s Sister” planet, because of the various things they have in common. For example, both planets reside within our Sun’s habitable zone (aka. “Goldilocks Zone“). In addition, Earth and Venus are also terrestrial planets, meaning they are primarily composed of metals and silicate rock that are differentiated between a metallic core and a silicate mantle and crust.
Beyond that, Earth and Venus could not be more different. And two ways in which they are in stark contrast is the time it takes for the Sun to rise, set, and return to the same place in the sky (i.e. one day). In Earth’s case, this process takes a full 24 hours. But in Venus’ case, its slow rotation and orbit mean that a single day lasts as long as 116.75 Earth days.
Sidereal Vs. Solar:
Naturally, some clarification is necessary when addressing the question of how long a day lasts. For starters, one must distinguish between a sidereal day and a solar day. A sidereal day is the time it takes for a planet to complete a single rotation on its axis. On the other hand, a solar day is the time it takes for the Sun to return to the same place in the sky.
On Earth, a sidereal days last 23 hours 56 minutes and 4.1 seconds, whereas a solar day lasts exactly 24 hours. In Venus’ case, it takes a whopping 243.025 days for the planet to rotate once on its axis – which is the longest rotational period of any planet in the Solar System. In addition, it rotates in the opposite the direction in which it orbits around the Sun (which it takes about 224.7 Earth days to complete).
In other words, Venus has a retrograde rotation, which means that if you could view the planet from above its northern polar region, it would be seen to rotate in a clockwise direction on its axis, and in a counter-clockwise direction around the Sun. It also means that if you could stand on the surface of Venus, the Sun would rise in the west and set in the east.
From all this, one might assume that a single day lasts longer than a year on Venus. But again, the distinction between a sidereal and solar days means that this is not true. Combined with its orbital period, the time it takes for the Sun to return to the same point in the sky works out to 116.75 Earth days, which is little more than a half a Venusian (or Cytherian) year.
Axial Tilt and Temperatures:
Unlike Earth or Mars, Venus has a very low axial tilt – just 2.64° relative to the ecliptic. In fact, it’s axial tilt is the one of the lowest in the Solar System, second only to Mercury (which has an extremely low tilt of 0.03°). Combined with its slow rotational period and dense atmosphere, this results in the planet being effectively isothermal, with virtually no variation in its surface temperature.
In other words, the planet experiences a mean temperature of 735 K (462 °C; 863.6 °F) – the hottest in the Solar System – with very little change between day and night, or between the equator and the poles. In addition, the planet experiences minimal seasonal temperature variation, with the only appreciable variations occurring with altitude.
It is a well-known fact that Venus’ atmosphere is incredibly dense. In fact, the mass of Venus atmosphere is 93 times that of Earth’s, and the air pressure at the surface is estimated to be as high as 92 bar – i.e. 92 times that of Earth’s at sea level. If it were possible for a human being to stand on the surface of Venus, they would be crushed by the atmosphere.
The composition of the atmosphere is extremely toxic, consisting primarily of carbon dioxide (96.5%) with small amounts of nitrogen (3.5%) and traces of other gases – most notably sulfur dioxide. Combined with its density, the composition generates the strongest greenhouse effect of any planet in the Solar System.
According to multiple Earth-based surveys and space missions to Venus, scientists have learned that its weather is rather extreme. The entire atmosphere of the planet circulates around quickly, with winds reaching speeds of up to 85 m/s (300 km/h; 186.4 mph) at the cloud tops, which circle the planet every four to five Earth days.
At this speed, these winds move up to 60 times the speed of the planet’s rotation, whereas Earth’s fastest winds are only 10-20% of the planet’s rotational speed. Spacecraft equipped with ultraviolet imaging instruments are able to observe the cloud motion around Venus, and see how it moves at different layers of the atmosphere. The winds blow in a retrograde direction, and are the fastest near the poles.
Closer to the equator, the wind speeds die down to almost nothing. Because of the thick atmosphere, the winds move much slower as you get close to the surface of Venus, reaching speeds of about 5 km/h. Because it’s so thick, though, the atmosphere is more like water currents than blowing wind at the surface, so it is still capable of blowing dust around and moving small rocks across the surface of Venus.
Venus flybys have also indicated that its dense clouds are capable of producing lightning, much like the clouds on Earth. Their intermittent appearance indicates a pattern associated with weather activity, and the lightning rate is at least half of that on Earth.
Yes, Venus is a planet of extremes. Extreme heat, extreme weather, and extremely long days! In short, there’s a reason why nobody lives there. But who knows? Given the right kind of technology, and perhaps even some dedicated terraforming efforts, people could one day being watching the Sun rising in the west and setting in the east.
Venus is often refereed to as “Earth’s sister planet”, thanks to the number of things it has in common with our planet. As a terrestrial planet, it is similarly composed of silicate rock and metals – which are differentiated between a metal core and a silicate crust and mantle. It also orbits within our Sun’s habitable zone, and had a similarly violent volcanic past.
But of course, there are also some major differences between our two planets. For one, Venus has an atmosphere that is incredibly dense (92 times that of Earth, in fact) and reaches temperatures that are hot enough to melt lead. In addition, the planet’s rotation is immensely slow by comparison, taking 243.025 days to complete a single rotation, and rotating backwards relative to Earth.
When discussing Venus’ rotation, it is important to note certain distinctions. Rotation is the time it takes for a planet to spin once on its axis. This is different from a planet’s revolution, which is the time it takes for a planet to orbit around another object (i.e. the Sun). So while it takes the Earth one day (24 hours) to rotate once on its axis, it takes one year (365.256 days) to revolve once around the Sun.
In Venus’ case, things work a little differently. For starters, it orbits the Sun at an average distance of about 0.72 AU (108,000,000 km; 67,000,000 mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System.
The planet completes a revolution around the Sun every 224.65 Earth days, which means that a year on Venus last about 61.5% as long as a year on Earth. Evey 584 days, Venus completes an interior conjunction, where it lies between Earth and the Sun. It is at this point that Venus makes the closest approach to Earth of any planet, at an average distance of 41 million km.
Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243.025 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a single day on Venus lasts longer than a Venusian year.
And, as noted earlier, Venus’ rotation is backwards, relative to Earth and the other bodies in the Solar System. Technically, this means that its rotational period is -243,025 days. It also means that if you could view the Solar System from the position above its celestial north pole, all of the planets (except for Uranus, which rotates on its side!) would appear to be rotating clockwise.
Venus, however, would appear to be rotating in a clockwise direction. Because of this, if you could stand on the surface of Venus, you would witness the Sun rising in the west and setting in the east. But you would be waiting a very long time to see this happen! Read on to find out why…
Sidereal vs. Solar Day:.
Another important thing to consider is the difference between a sidereal day and a solar day. A sidereal day corresponds to the amount of time it takes for a planet to rotate once on its axis, which in Venus’ case takes 243.025 Earth days. A solar day, by contrast, refers to the amount of time it takes for the Sun to reappear at the same point in the sky (i.e. between one sunrise/sunset and the next).
A Venusian (aka. Cytherean) Solar Day is the equivalent to 116.75 days on Earth, which means that it takes almost 117 days for the sun to rise, set, and return to the same place in the sky. Doing the math, we then see that a single year on Venus (224.65 Earth days) works out to just 1.92 Venusian (solar) days. Not exactly the basis for a good calendar system, is it?
Yes, when it comes to the planet Venus, things work quite differently than they do here on Earth. Not only does a day last over half a year on our “Sister Planet”, but the Sun rises and sets on the opposite horizons, and travels across the sky in the opposite direction. The reason for this, according to astronomers, is that billions of years ago (early in the planet’s history) Venus was impacted by another large planet.
The combined momentum between the two objects averaged out to the current rotational speed and direction, causing Venus to spin very slowly in its current retrograde motion. Someday, if human beings colonize there (perhaps in floating cities) they will have to learn to get used to a day that lasts over 2800 Earth hours, not to mention sunrises and sunsets happening on the wrong horizon!
You may have heard the saying at some point in your life: “The Sun will still rise in the east and set in the west tomorrow.” You get the point, it means it’s not the end of the world. But have you ever wondered why the Sun behaves this way? Why does – and always has, for that matter – the Sun rise in the east and set in the west? What mechanics are behind this?
Naturally, ancient people took the passage of the Sun through the sky as a sign that it was revolving around us. With the birth of modern astronomy, we have come to learn that its actually the other way around. The Sun only appears to be revolving around us because our planet not only orbits it, but also rotates on its axis as it is doing so. From this, we get the familiar passage of the Sun through the sky, and the basis for our measurement of time.
As already noted, the Earth rotates on its axis as it circles the Sun. If viewed from above the celestial north, the Earth would appear to be rotating counter-clockwise. Because of this, to those standing on the Earth’s surface, the Sun appears to be moving around us in a westerly direction at a rate of 15° an hour (or 15′ a minute). This is true of all celestial objects observed in the sky, with an “apparent motion” that takes them from east to west.
This is also true of the majority of the planets in the Solar System. Venus is one exception, which rotates backwards compared to its orbit around the Sun (a phenomena known as retrograde motion). Uranus is another, which not only rotates westward, but is inclined so much that it appears to be sitting on its side relative to the Sun.
Pluto also has a retrograde motion, so for those standing on its surface, the Sun would rise in the west and set in the east. In all cases, a large impact is believed to be the cause. In essence, Pluto and Venus were sent spinning in the other direction by a large impact, while another struck Uranus and knocked it over on its side!
With a rotational velocity of 1,674.4 km/h (1,040.4 mph), the Earth takes 23 hours, 56 minutes and 4.1 seconds to rotate once on its axis. This means, in essence, that a sidereal day is less than 24 hours. But combined with its orbital period (see below), a solar day – that is, the time it takes for the Sun to return to the same place in the sky – works out to 24 hours exactly.
Earth’s Orbit Around the Sun:
With an average orbital velocity of 107,200 km/h (66,600 mph), the Earth takes approximately 365.256 days – aka. a sidereal year – to complete a single orbit of the Sun. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day.
Viewed from the celestial north, the motion of the Earth appears to orbit the Sun in a counterclockwise direction. Combined with its axial tilt – i.e. the Earth’s axis is tilted 23.439° towards the ecliptic – this results in seasonal changes. In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Basically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. During the summer, the climate warms up and the sun appears earlier in the morning sky and sets at a later hour in the evening. In the winter, the climate becomes generally cooler and the days are shorter, with sunrise coming later and sunset happening sooner.
Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, which is known as a “polar night”. In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a “midnight sun” – i.e. a day of 24 hours.
And last, but not least, seasonal changes also result in changes in the Sun’s apparent motion across the sky. During summer in the northern hemisphere, the Sun appears to move from east to west directly overhead, while moving closer to the southern horizon during winter. During summer in the southern hemisphere, the Sun appears to move overhead; while in the winter, it appears to be closer to the northern horizon.
In short, the Sun rises in the east and sets in the west because of our planet’s rotation. During the course of the year, the amount of daylight we experience is mitigated by our planet’s tilted axis. If, like Venus, Uranus and Pluto, a large enough asteroid or celestial object were to strike us just right, the situation might be changed. We too could experience what it is like to watch the Sun rise in the west, and set in the east.
But when it comes to things like the length of a year, and the length of seasons, Mars and Earth are quite different. Compared to Earth, a year on Mars lasts almost twice as long – 686.98 Earth days. This is due to the fact that Mars is significantly farther from the Sun and its orbital period (the time it takes to orbit the Sun) is significantly greater than that of Earth’s.
Mars average distance (semi-major axis) from the Sun is 227,939,200 km (141,634,852.46 mi) which is roughly one and half times the distance between the Earth and the Sun (1.52 AU). Compared to Earth, its orbit is also rather eccentric (0.0934 vs. 0.0167), ranging from 206.7 million km (128,437,425.435 mi; 1.3814 AU) at perihelion to 249.2 million km (154,845,701 mi; 1.666 AU) at aphelion. At this distance, and with an orbital speed of 24.077 km/s, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a orbit around the Sun.
This eccentricity is one of the most pronounced in the Solar System, with only Mercury having a greater one (0.205). However, this wasn’t always the case. Roughly 1.35 million years ago, Mars had an eccentricity of just 0.002, making its orbit nearly circular. It reached a minimum eccentricity of 0.079 some 19,000 years ago, and will peak at about 0.105 in about 24,000 years from now.
But for the last 35,000 years, the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other planets. The closest distance between Earth and Mars will continue to mildly decrease for the next 25,000 years. And in about 1,000,000 years from now, its eccentricity will once again be close to what it is now – with an estimated eccentricity of 0.01.
Earth Days vs. Martian “Sols”:
Whereas a year on Mars is significantly longer than a year on Earth, the difference between an day on Earth and a Martian day (aka. “Sol”) is not significant. For starters, Mars takes 24 hours 37 minutes and 22 seconds to complete a single rotation on its axis (aka. a sidereal day), where Earth takes just slightly less (23 hours, 56 minutes and 4.1 seconds).
On the other hand, it takes 24 hours, 39 minutes, and 35 seconds for the Sun to appear in the same spot in the sky above Mars (aka. a solar day), compared to the 24 hour solar day we experience here on Earth. This means that, based on the length of a Martian day, a Martian year works out to 668.5991 Sols.
Mars also has a seasonal cycle that is similar to that of Earth’s. This is due in part to the fact that Mars also has a tilted axis, which is inclined 25.19° to its orbital plane (compared to Earth’s axial tilt of approx. 23.44°). It’s also due to Mars orbital eccentricity, which means it will periodically receive less in the way of the Sun’s radiance during at one time of the year than another. This change in distance causes significant variations in temperature.
While the planet’s average temperature is -46 °C (51 °F), this ranges from a low of -143 °C (-225.4 °F) during the winter at the poles to a high of 35 °C (95 °F) during summer and midday at the equator. This works out to a variation in average surface temperature that is quite similar to Earth’s – a difference of 178 °C (320.4 °F) versus 145.9 °C (262.5 °F). This high in temperatures is also what allows for liquid water to still flow (albeit intermittently) on the surface of Mars.
In addition, Mars’ eccentricity means that it travels more slowly in its orbit when it is further from the Sun, and more quickly when it is closer (as stated in Kepler’s Three Laws of Planetary Motion). Mars’ aphelion coincides with Spring in its northern hemisphere, which makes it the longest season on the planet – lasting roughly 7 Earth months. Summer is second longest, lasting six months, while Fall and Winter last 5.3 and just over 4 months, respectively.
In the south, the length of the seasons is only slightly different. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder. The summer temperatures in the south can be up to 30 K (30 °C; 54 °F) warmer than the equivalent summer temperatures in the north.
These seasonal variations allow Mars to experience some extremes in weather. Most notably, Mars has the largest dust storms in the Solar System. These can vary from a storm over a small area to gigantic storms (thousands of km in diameter) that cover the entire planet and obscure the surface from view. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.
The first mission to notice this was the Mariner 9 orbiter, which was the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.
And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.
These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, leading to Mars’ experiencing its own greenhouse effect.
Given the differences in seasons and day length, one is left to wonder if a standard Martian calendar could ever be developed. In truth, it could, but it would be a bit of a challenge. For one, a Martian calendar would have to account for Mars’ peculiar astronomical cycles, and our own non-astronomical cycles like the 7-day week work with them.
Another consideration in designing a calendar is accounting for the fractional number of days in a year. Earth’s year is 365.24219 days long, and so calendar years contain either 365 or 366 days accordingly. Such a formula would need to be developed to account for the 668.5921-sol Martian year. All of this will certainly become an issue as human beings become more and more committed to exploring (and perhaps colonizing) the Red Planet.
I’m going to ask you how long a day is on Earth, and you’re going to get the haunting suspicion that this is a trap. Your instincts are right, it’s a trap! The answer may surprise you.
How long is a day on Earth? Or more specifically, how long does it take for the Earth to turn once on its axis? For all the stars to move through the sky and return to their original position? Go ahead, and yell your answer answer at the screen… 24 hours?
Wrong! It only takes 23 hours, 56 minutes and 4.0916 seconds for the Earth to turn once its axis. Unless that’s what you said. In which case, congratulations!
I’m sure you’re now stumbling around in an incoherent state, trying to understand how you could have possibly messed this up. Were you reprogrammed by the hidden chronology conspiracy? Have time travellers been setting back all your clocks every day by 4 minutes? How was your whole life a lie?
Here’s the deal. When you consider a day, you’re probably thinking of your trusty clock, or maybe that smartphone lock screen that clearly measures 24 hours.
What you have come to understand as a “day” is classified by astronomers as a solar day. It’s the amount of time it takes for the Sun to move through the sky and return to roughly the same spot.
This is different from the amount of time it takes for the Earth to turn once on its axis – the 23 hours, 56 minutes. Also known as a sidereal day.
Why are these two numbers different? Imagine the Earth orbiting the Sun, taking a full 365 days, 5 hours, 48 minutes and 46 seconds to complete the entire journey. At the same time, the Earth is spinning on its axis.
Each day that goes by, the Earth needs to turn a little further for the Sun to return to the same place in the sky.… And that extra time is about 4 minutes.
If we only measured sidereal days, the position of the Sun would slip back, day after day. For half of the year, the Sun would be up between 12am and 12pm, and for the other half, it would be between 12pm and 12am. There would be no connection between what time it is, and whether or not the Sun is in the sky.
Can you imagine teaching your children how to read a clock, and then getting them to multiply that by the calendar to figure out when My Little Pony: Friendship is Magic starts? Madness.
Better to keep them in the dark, teach them that a day is 24 hours, and deny all knowledge when they get a little older, and start to ask you challenging questions. But pedants among you already knew that, didn’t you?
You already knew that a sidereal day is a little shorter than a solar day, and that everyone else has been living a lie. You’re the only one who can read the signs and know the terrifying truth. Aren’t you? Well, I’m here to tell you that you’re wrong too. There’s a deeper conspiracy that you’re not a part of. Dear Pedant, your life is also a lie.
The axis of the Earth’s pole, the imaginary line that you could draw between the south pole and the north pole is currently pointed roughly at Polaris, aka The North Star. But we’re wobbling like a top, and where the axis is pointing is slowly precessing westward over the course of 26,000 years. This means that a sidereal day is actually 0.0084 seconds shorter when you account for this extra movement of the Earth’s axis.
There are other events that can increase or decrease the length of an Earth day. Because of our tidal interactions with the Moon, the length of a day on Earth has increased by about 1.7 milliseconds over the last 100 years. Powerful earthquakes can change the Earth’s rotation time by a few microseconds depending on how the tectonic plates shove around. Even as the glaciers melt, the rotation speed slows down a little more.
So, if someone asks you how long a day is, make sure they clarify whether it’s a solar day or a sidereal day. And then ask if they’d like you to incorporate the Earth’s precession, tidal locking and recent earthquakes into the calculation.
If they give you a knowing nod, congratulations, you’re talking to another member of the vast chronology conspiracy.
When did you discover your whole life was a lie? Tell us in the comments below.