How Long is a Day on Earth?

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.

Axis of the Earth’s pole. Credit: NASA / Mysid
Axis of the Earth’s pole. Credit: NASA / Mysid

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.

Earthquakes can change the rotation of the Earth. Credit: USGS / Google Maps / AJAX / SODA
Earthquakes. Credit: USGS / Google Maps / AJAX / SODA

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.

What Are The Most Famous Stars?

While there are untold billions of celestial objects visible in the nighttime sky, some of them are better known than others. Most of these are stars that are visible to the naked eye and very bright compared to other stellar objects. For this reason, most of them have a long history of being observed and studied by human beings, and most likely occupy an important place in ancient folklore.

So without further ado, here is a sampling of some of the better-known stars in that are visible in the nighttime sky:

Polaris:
Also known as the North Star (as well as the Pole Star, Lodestar, and sometimes Guiding Star), Polaris is the 45th brightest star in the night sky. It is very close to the north celestial pole, which is why it has been used as a navigational tool in the northern hemisphere for centuries. Scientifically speaking, this star is known as Alpha Ursae Minoris because it is the alpha star in the constellation Ursa Minor (the Little Bear).

The Polaris star system, as seen within the Ursa Minor constellation and up close. Credit: NASA, ESA, N. Evans (Harvard-Smithsonian CfA), and H. Bond (STScI)
The Polaris star system, as seen within the Ursa Minor constellation and up close. Credit: NASA, ESA, N. Evans (Harvard-Smithsonian CfA), and H. Bond (STScI)

It’s more than 430 light-years away from Earth, but its luminosity (being a white supergiant) makes it highly visible to us here on Earth. What’s more, rather than being a single supergiant, Polaris is actually a trinary star system, comprised of a main star (alpha UMi Aa) and two smaller companions (alpha UMi B, alpha UMi Ab). These, along with its two distant components (alpha UMi C, alpha UMi D), make it a multistar system.

Interestingly enough, Polaris wasn’t always the north star. That’s because Earth’s axis wobbles over thousands of years and points in different directions. But until such time as Earth’s axis moves farther away from the “Polestar”, it remains our guide.

Because it is what is known as a Cepheid variable star – i.e. a star that pulsates radially, varying in both temperature and diameter to produce brightness changes – it’s distance to our Sun has been the subject of revision. Many scientific papers suggest that it may be up to 30% closer to our Solar System than previously expected – putting it in the vicinity of 238 light years away.

Time exposure centered on Polaris, the North Star. Notice that the closer stars are to Polaris, the smaller the circles they describe. Stars at the edge of the frame make much larger circles. Credit: Bob King
Time exposure centered on Polaris, the North Star. Notice that the closer stars are to Polaris, the smaller the circles they describe. Stars at the edge of the frame make much larger circles. Credit: Bob King

Sirius:
Also known as the Dog Star, because it’s the brightest star in Canis Major (the “Big Dog”), Sirius is also the brightest star in the night sky. The name “Sirius” is derived from the Ancient Greek “Seirios“, which translates to “glowing” or “scorcher”. Whereas it appears to be a single bright star to the naked eye, Sirius is actually a binary star system, consisting of a white main-sequence star named Sirius A, and a faint white dwarf companion named Sirius B.

The reason why it is so bright in the sky is due to a combination of its luminosity and distance – at 6.8 light years, it is one of Earth’s nearest neighbors. And in truth, it is actually getting closer. For the next 60,000 years or so, astronomers expect that it will continue to approach our Solar System; at which point, it will begin to recede again.

In ancient Egypt, it was seen as a signal that the flooding of the Nile was close at hand. For the Greeks, the rising of Sirius in the night sky was a sign of the”dog days of summer”. To the Polynesians in the southern hemisphere, it marked the approach of winter and was an important star for navigation around the Pacific Ocean.

Alpha Centauri System:
Also known as Rigel Kent or Toliman, Alpha Centauri is the brightest star in the southern constellation of Centaurus and the third brightest star in the night sky. It is also the closest star system to Earth, at just a shade over four light-years. But much like Sirius and Polaris, it is actually a multistar system, consisting of Alpha Centauri A, B, and Proxima Centauri (aka. Centauri C).

Artist’s impression of the planet around Alpha Centauri B. Credit: ESO
Artist’s impression of the planet around Alpha Centauri B. Credit: ESO

Based on their spectral classifications, Alpha Centauri A is a main sequence white dwarf with roughly 110% of the mass and 151.9% the luminosity of our Sun. Alpha Centauri B is an orange subgiant with 90.7% of the Sun’s mass and 44.5% of its luminosity. Proxima Centauri, the smallest of the three, is a red dwarf roughly 0.12 times the mass of our Sun, and which is the closest of the three to our Solar System.

English explorer Robert Hues was the first European to make a recorded mention of Alpha Centauri, which he did in his 1592 work Tractatus de Globis. In 1689, Jesuit priest and astronomer Jean Richaud confirmed the existence of a second star in the system. Proxima Centauri was discovered in 1915 by Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa.

In 2012, astronomers discovered an Earth-sized planet around Alpha Centauri B. Known as Alpha Centauri Bb, it’s close proximity to its parent star likely means that it is too hot to support life.

Betelgeuse:
Pronounced “Beetle-juice” (yes, the same as the 1988 Tim Burton movie), this bright red supergiant is roughly 65o light-year from Earth. Also known as Alpha Orionis, it is nevertheless easy to spot in the Orion constellation since it is one of the largest and most luminous stars in the night sky.

Betelgeuse, as seen by the Hubble Space Telescope. Credit: NASA
Betelgeuse, as seen by the Hubble Space Telescope, and in relation to the Orion constellation. Credit: NASA

The star’s name is derived from the Arabic name Ibt al-Jauza’, which literally means “the hand of Orion”. In 1985, Margarita Karovska and colleagues from the Harvard–Smithsonian Center for Astrophysics, announced the discovery of two close companions orbiting Betelgeuse. While this remains unconfirmed, the existence of possible companions remains an intriguing possibility.

What excites astronomers about Betelgeuse is it will one day go supernova, which is sure to be a spectacular event that people on Earth will be able to see. However, the exact date of when that might happen remains unknown.

Rigel:
Also known as Beta Orionis, and located between 700 and 900 light years away, Rigel is the brightest star in the constellation Orion and the seventh brightest star in the night sky. Here too, what appears to be a blue supergiant is actually a multistar system. The primary star (Rigel A) is a blue-white supergiant that is 21 times more massive than our sun, and shines with approximately 120,000 times the luminosity.

Rigel B is itself a binary system, consisting of two main sequence blue-white subdwarf stars. Rigel B is the more massive of the pair, weighing in at 2.5 Solar masses versus Rigel C’s 1.9. Rigel has been recognized as being a binary since at least 1831 when German astronomer F.G.W. Struve first measured it. A fourth star in the system has been proposed, but it is generally considered that this is a misinterpretation of the main star’s variability.

Rigel A is a young star, being only 10 million years old. And given its size, it is expected to go supernova when it reaches the end of its life.

Vega:
Vega is another bright blue star that anchors the otherwise faint Lyra constellation (the Harp). Along with Deneb (from Cygnus) and Altair (from Aquila), it is a part of the Summer Triangle in the Northern hemisphere. It is also the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere (after Arcturus).

Characterized as a white dwarf star, Vega is roughly 2.1 times as massive as our Sun. Together with Arcturus and Sirius, it is one of the most luminous stars in the Sun’s neighborhood. It is a relatively close star at only 25 light-years from Earth.

Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded. It was also one of the first stars whose distance was estimated through parallax measurements, and has served as the baseline for calibrating the photometric brightness scale. Vega’s extensive history of study has led it to be termed “arguably the next most important star in the sky after the Sun.”

Artist's concept of a recent massive collision of dwarf planet-sized objects that may have contributed to the dust ring around the star Vega. Credit: NASA/JPL/Caltech/T. Pyle (SSC)
Artist’s concept of a recent massive collision of dwarf planet-sized objects that may have contributed to the dust ring around the star Vega. Credit: NASA/JPL/Caltech/T. Pyle (SSC)

Based on observations that showed excess emission of infrared radiation, Vega is believed to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in an orbiting debris disk. For this reason, stars that display an infrared excess because of circumstellar dust are termed “Vega-like stars”.

Thousands of years ago, (ca. 12,000 BCE) Vega was used as the North Star is today, and will be so again around the year 13,727 CE.

Pleiades:
Also known as the “Seven Sisters”, Messier 45 or M45, Pleiades is actually an open star cluster located in the constellation of Taurus. At an average distance of 444 light years from our Sun, it is one of the nearest star clusters to Earth, and the most visible to the naked eye. Though the seven largest stars are the most apparent, the cluster actually consists of over 1,000 confirmed members (along with several unconfirmed binaries).

The core radius of the cluster is about 8 light years across, while it measures some 43 light years at the outer edges. It is dominated by young, hot blue stars, though brown dwarfs – which are just a fraction of the Sun’s mass – are believed to account for 25% of its member stars.

Pleiades by Jamie Ball
Pleiades, also known as M45, is a prominent open star cluster in the sky. Image Credit: Jamie Ball

The age of the cluster has been estimated at between 75 and 150 million years, and it is slowly moving in the direction of the “feet” of what is currently the constellation of Orion. The cluster has had several meanings for many different cultures here on Earth, which include representations in Biblical, ancient Greek, Asian, and traditional Native American folklore.

Antares:
Also known as Alpha Scorpii, Antares is a red supergiant and one of the largest and most luminous observable stars in the nighttime sky. It’s name – which is Greek for “rival to Mars” (aka. Ares) – refers to its reddish appearance, which resembles Mars in some respects. It’s location is also close to the ecliptic, the imaginary band in the sky where the planets, Moon and Sun move.

This supergiant is estimated to be 17 times more massive, 850 times larger in terms of diameter, and 10,000 times more luminous than our Sun. Hence why it can be seen with the naked eye, despite being approximately 550 light-years from Earth. The most recent estimates place its age at 12 million years.

A red supergiant, Antares is about 850 times the diameter of our own Sun, 15 times more massive, and 10,000 times brighter. Credit: NASA/Ivan Eder
A red supergiant, Antares is over 850 times the diameter of our own Sun, 15 times more massive, and 10,000 times brighter. Credit: NASA/Ivan Eder

Antares is the seventeenth brightest star that can be seen with the naked eye and the brightest star in the constellation Scorpius. Along with Aldebaran, Regulus, and Fomalhaut, Antares comprises the group known as the ‘Royal stars of Persia’ – four stars that the ancient Persians (circa. 3000 BCE) believed guarded the four districts of the heavens.

Canopus:
Also known as Alpha Carinae, this white giant is the brightest star in the southern constellation of Carina and the second brightest star in the nighttime sky. Located over 300 light-years away from Earth, this star is named after the mythological Canopus, the navigator for king Menelaus of Sparta in The Iliad. 

Thought it was not visible to the ancient Greeks and Romans, the star was known to the ancient Egyptians, as well as the Navajo, Chinese and ancient Indo-Aryan people. In Vedic literature, Canopus is associated with Agastya, a revered sage who is believed to have lived during the 6th or 7th century BCE. To the Chinese, Canopus was known as the “Star of the Old Man”, and was charted by astronomer Yi Xing in 724 CE.

An image of Canopus, as taken by crewmembers aboard the ISS. Credit: NASA
Image of Canopus, as taken by crew members aboard the ISS. Credit: NASA

It is also referred to by its Arabic name Suhayl (Soheil in persian), which was given to it by Islamic scholars in the 7th Century CE. To the Bedouin people of the Negev and Sinai, it was also known as Suhayl, and used along with Polaris as the two principal stars for navigation at night.

It was not until 1592 that it was brought to the attention of European observers, once again by Robert Hues who recorded his observations of it alongside Achernar and Alpha Centauri in his Tractatus de Globis (1592).

As he noted of these three stars, “Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third is in the right foote of the Centaure.”

This star is commonly used for spacecraft to orient themselves in space, since it is so bright compared to the stars surrounding it.

Universe Today has articles on what is the North Star and types of stars. Here’s another article about the 10 brightest stars. Astronomy Cast has an episode on famous stars.

Star Trail Photo Hints at Hidden Polestars

A week ago I made a 45-minute time exposure of the southern sky featuring the planet Mars. As the Earth rotated on its axis, the stars trailed across the sky. But take a closer look at the photo and you’ll see something interesting going on. 

The trails across the diagonal (upper right to lower left) are straight, those in the top third arc upward or north while those in the bottom third curve downward or south.

I've drawn part of the imaginary great circle in the sky called the celestial equator. Residents of cities on or near the Earth's equator see the celestial equator pass directly overhead. From mid-northern latitudes, it's about halfway up in the southern sky. From mid-southern latitudes, it's halfway up in the northern sky. Credit: Bob King
I’ve drawn part of the imaginary great circle in the sky called the celestial equator. Residents of cities on or near the Earth’s equator see the celestial equator pass directly overhead. From mid-northern latitudes, it’s about halfway up in the southern sky. From mid-southern latitudes, it’s halfway up in the northern sky. Credit: Bob King

I suspect you know what’s happening here. Mars happens to lie near the celestial equator, an extension of Earth’s equator into the sky. The celestial equator traces a great circle around the celestial sphere much as the equator completely encircles the Earth.

On Earth, cities north of the equator are located in the northern hemisphere, south of the equator in the southern hemisphere. The same is true of the stars. Depending on their location with respect to the celestial equator they belong either to the northern or southern halves of the sky.

Earth's axis points north to Polaris, the northern hemisphere's North Star, and south to dim Sigma Octantis. Illustration: Bob King
Earth’s axis points north to Polaris, the northern hemisphere’s North Star, and south to dim Sigma Octantis. Illustration: Bob King

Next, let’s take a look at Earth’s axis and where each end points. If you live in the northern hemisphere, you know that the axis points north to the North Star or Polaris. As the Earth spins, Polaris appears fixed in the north while all the stars in the northern half of the sky describe a circle around it every 24 hours (one Earth spin). The closer a star is to Polaris, the tighter the circle it describes.

Time exposure centered on Polaris, the North Star. Notice that the closer stars are to Polaris, the smaller the circles they describe. Stars at the edge of the frame make much larger circles. Credit: Bob King
Time exposure centered on Polaris, the North Star. Notice that the closer stars are to Polaris, the smaller the circles they describe. Stars at the edge of the frame make much larger circles. Credit: Bob King

Likewise, from the southern hemisphere, all the southern stars circle about the south pole star, an obscure star named Sigma in the constellation of Octans, a type of navigational instrument. Again, as with Polaris, the closer a star lies to Sigma Octantis, the smaller its circle.

Stars trail around the dim southern pole star Sigma Octantis as seen from the southern hemisphere. The two smudges are the Large and Small Magellanic Clouds, companion galaxies of the Milky Way. Credit: Ted Dobosz
Stars trail around the dim southern pole star Sigma Octantis as seen from the southern hemisphere. The two smudges are the Large and Small Magellanic Clouds, companion galaxies of the Milky Way. Credit: Ted Dobosz

But what about stars on or near the celestial equator? These gems are the maximum distance of 90 degrees from either pole star just as Earth’s equator is 90 degrees from the north and south poles. They “tread the line” between both hemispheres and make circles so wide they appear not as arcs – as the other stars do in the photo – but as straight lines. And that’s why stars appear to be heading in three separate directions in the photograph.

A view of the entire sky as seen from Quito, Ecuador on the equator this evening. The celestial equator crosses directly overhead while each pole star lies 90 degrees away on opposite horizons. Stellarium
A view of the entire sky as seen from Quito, Ecuador on the equator this evening. The celestial equator crosses directly overhead while each pole star lies 90 degrees away on opposite horizons. Stellarium

In so many ways, we see aspects of our own planet in the stars above.

The Little Cepheid that Stopped

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When Hubble first discovered a Cepheid variable in the galaxy M31, the universe grew. Previously, many astronomers had held that the fuzzy “spiral nebulae” were small patches of gas and dust within our own galaxy, but through the Period-Luminosity relationship which allowed him to determine the distance, Hubble demonstrated that these were “island universes”, or galaxies in their own right.

Soon after, Hubble (as well as other astronomers) began searching other fuzzy patches for Cepheids. Among them was the spiral galaxy M33 in which he discovered 35 Cepheids. Among them was V19 which had a 54.7 day period, an average magnitude of 19.59 ± 0.23 MB, and an amplitude of 1.1 magnitudes. But according to recent work revealed at the recent American Astronomical Society meeting, V19 no longer seems to be pulsating as a Cepheid.

The new research uses observations from the 3.5m Wisconsin, Indiana, Yale, and NOAO (WIYN) Observatory as well as the 1.3m Robotically Controlled Telescope (RCT) operated jointly by a group of universities and research institutions. The new observations confirm a 2001 report that found V19 had decreased its brightness amplitude to at least less than 10% of the magnitude reported by Hubble in 1926, and possibly further as any fluctuations were below the threshold detectable by the instruments.

Now, if any variation exists, it is less than 0.1 magnitudes. The new study reports that there may be some small fluctuations, but due to inherent uncertainty in the observations, it barely exceeds the background noise and the announcers did not commit to these findings. Instead, they pledged to continue observations with larger instruments to the equation to push down the instrumental error as well as adding spectroscopic measurements to investigate other changes in the star. Another of the peculiar changes V19 has undergone is an increase of about half of a magnitude to 19.08 ± 0.05.

These changes are strikingly similar to another, more famous star: Polaris. Due to its much closer nature, observations have been much more frequent and with lower detection thresholds. This star had previously been reported to have an amplitude of 0.1 magnitudes which, according to a 2004 study, had decreased to 0.03 magnitudes. Additionally, based on ancient records, astronomers have estimated that Polaris has also brightened about a full magnitude in the past 2,000 years.

According to Edward Guinan of Villanova University and one of the members of the new observational team, “both stars are experiencing unexpectedly fast and large changes in their pulsation properties and brightness that are not yet explained by theory.”

The primary explanation for this dramatic change is simple evolution: As the stars have aged, they have moved out of the instability strip, a region on the HR diagram in which stars are prone to pulsations. But these stars may not be entirely lost from the family of periodic variables. In 2008, a study led by Hans Bruntt of the University of Sidney suggested that Polaris’ amplitude may be increasing. The team found that from 2003 to 2006, the scale of the oscillations had increased by 30%.

This has led other astronomers to suspect that there may be an additional effect in play in Cepheids known as the Blazhko Effect. This effect, often seen in RR Lyrae stars (another type of periodic variables), is a periodic variation of the variation. While no firm explanation exists for this effect, astronomers have suggested that it may be due to multiple pulsational modes that interfere constructively and destructively and occasionally forming resonances.

Ultimately, these strange changes in brightness are unexplained and will require astronomers to have to carefully monitor these stars, as well as other Cepheids to search for causes.