Universe Today

June 16, 2008December 24, 2015 by Tammy Plotner

Simeis 147 by Davide De Martin

If you think we’re looking straight down the maul of the “Doomsday Machine”, you’d be pretty much correct. While the fictionalized Star Trek account had the planet killer slowly destroying a distant solar system, this particular “star eater” is very real and still exists along the Auriga-Taurus border…

Named Simeis 147, this ancient supernova remnant has expanded so much that it’s barely visible to larger telescopes. Why? Mostly because the diameter of the nebula is about 3-1/2 degrees, or about 7 times the size of the Moon – and the fact it’s one of the faintest objects in the night sky. Like many nebulous “sky scraps”, it is simply too large to be seen in its entirety – or beauty – except through the magic of astrophotograhy.

In this week’s image by Davide De Martin, we take an up close and personal look at Simeis 147. The intricate filaments of this faint supernova remnant spans over 160 light years of interstellar space and is around 3900 light years away. With an apparent age of about 100,000 years, this awesome explosion occurred around the time of Peking Man, and like our distant ancestor left more than one artifact behind. In this case, the expanding remnant is not all. Deep within the folds and rifts lay a spinning neutron star. This pulsar is all that’s left of the original star’s core.

Unlike many things unexplored, more study was indicated and newer estimated gauge Semeis 147’s age at about 30,000 years. The pulsar itself has recently been detected and has been cataloged as PSR J0538+2817. Imagine something that rotates completely on its axis seven times per second! And think about what happened… The outer layers of this exploding star initially carried outward at speeds of 10,000-20,000 km/s–a tremendous amount of energy released in a blast wave.

Supernovae are divided into classes based upon the appearance of their spectra: hydrogen lines are prominent in Type II supernovae; while hydrogen lines are absent in Type Ia supernovae. Put simply, this means the progenitor stars either had hydrogen in their outer envelopes or did not have hydrogen in their outer envelopes. Type II supernovae are the territory of massive stars while Type Ia supernovae more than likely originated with white dwarf binary star systems – a place where the accreting white dwarf is driven above the Chandrasekhar Mass Limit, collapses and explodes.

So how often do events like the Simeis 147 type happen? According to Rudolph Minkowski; “As regards the supernovae frequency, there are two types of supernovae. The Supernovae I seem to occur about every 400 or 500 years per galaxy and the Supernovae II about every 50 years per galaxy, with considerable leeway. But, the Supernovae II are certainly much more frequent than Supernova I.” In recent studies done the 610.5 MHz Contour Maps of the Supernova Simeis 147, by Dickel and McKinley, the integrated flux densities show that the radiation is probably non-thermal and incredibly old.

As old as the Star Trek “Doomsday Machine”? Its origins were also unknown and it produced mass destruction. Maybe Simeis 147 isn’t quite the same as the neutronium hulled, antiproton beam firing planet killer of Gene Roddenberry’s fictionalized story… But it is definitely as intriguing to the imagination!

This week’s awesome image was done by Davide De Martin.

June 16, 2008December 24, 2015 by Jerry Coffey

How Long is a Year on Jupiter

The answer to ”how long is a year on Jupiter” is 11.86 Earth years. There is so much more to know about the Jovian system, that we can not just leave you with one fact, so here are some more interesting facts about Jupiter.

At perihelion Jupiter is 741 million km from the Sun(4.95 AU). At aphelion it is 817 million km from the Sun(5.46 AU). That gives Jupiter a semi-major axis of 778,340,821 km. Jupiter’s orbit varies by 76 million km, but it has one of the least eccentric orbits in the Solar System.

Jupiter has 2.5 times the mass of all of the other objects in the Solar System except the Sun. It is so massive that if it gained any more mass it would shrink. Gravitational compression would take over making the planet more dense instead of larger.

There are some conspiracy theorists who like to propose that Jupiter will become a star and destroy Earth. That can never happen. Jupiter would have to accrete about 80 times more mass than it has now and experience a huge increase in temperature in order to ignite fusion. The planet has the hydrogen it needs, but not the wherewithal to fuse it into helium and become a star.

Earth’s magnetic field is generated by its core through a dynamo effect. Scientist are not even sure that Jupiter has a rocky/metallic core, yet the planet has a magnetic field that is 14 times stronger than Earth’s. Astronomers think the magnetic field is generated by the churning of metallic hydrogen near the center of Jupiter. This magnetic field traps ionized particles from the solar wind and accelerates them to nearly the speed of light.

One of the most well known aspects of Jupiter is the Great Red Spot. Astronomers have been documenting it for nearly 350 years. It seems to grow and shrink over time. It is actually a giant storm that would totally engulf the Earth. At one time the storm covered an area that was 40,000 km long. It is slowly getting smaller, but astronomers do not know if it will ever disappear.

Knowing the answer to ”how long is a year on Jupiter” is just one minor detail about the planet. The others above are just a few facts that do not even scratch the surface of the Jovian mystery. None of Jupiter’s 67 moons or it ring system have been mentioned. Imagine the stories yet to be told.

Here’s a great image of Jupiter, captured by amateur astronomer Mike Salway, and an interesting hypothetical article about how Jupiter’s orbit could mess up the Solar System.

Here’s some general information on Jupiter from the Nine Planets, and more information from Solar Views.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.

Sources:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=OverviewLong
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=Facts

June 16, 2008December 24, 2015 by Jerry Coffey

How Long is a Day on Jupiter

The Universe Today readers are always asking great questions. ”How long is a day on Jupiter?”, is one of them. A day on Jupiter, also known as the sidereal rotation period, lasts 9.92496 hours. Jupiter is the fastest rotating body in our Solar System. Determining the length of a day on Jupiter was very difficult, because, unlike the terrestrial planets, it does not have surface features that scientists could use to determine its rotational speed.

Scientists cast about for ways to judge the planet’s rotational speed. An early attempt was to do some storm watching. Jupiter is constantly buffeted by atmospheric storms, so the theory was that you could locate the center of a storm and get some idea of the length of a day. The problem scientists encountered was that the storms on Jupiter are very fast moving, making them an inaccurate source of rotational information. Scientist were finally able to use radio emissions from Jupiter’s magnetic field to calculate the planet’s rotational period and speed. While other parts of the planet rotate at different speeds, the speed as measured by the magnetosphere is used as the official rotational speed and period.

All of the planets are oblate spheroids with varying degrees of flattening. Jupiter’s extremely fast rotation flattens it more than any other planet. The diameter of the equator is 9275 km more than the distance from pole to pole. Another interesting effect of Jupiter’s rotational speed is that, because Jupiter is not a solid body, its upper atmosphere features differential rotation. The atmosphere above the poles rotates about five minutes slower than the atmosphere at the equator.

Jupiter is almost a solar system unto itself. Many astronomers believe the the planet is simply a failed star, just lacking the mass needed to ignite fusion. Many people are aware of its four largest moons, the Galilean moons Io, Europa, Ganymede, and Callisto, but few realize that Jupiter has 50 confirmed moons and at least 14 provisional moons. The four largest moons are all very interesting to scientists. Io is a volcanic nightmare. Europa is covered in water ice and may have oceans of slushy ice underneath. Ganymede is the largest moon in the Solar System, even bigger than Mercury, and is the only moon known to have an internally generated magnetic field like Earth’s. Callisto is interesting because its surface is thought to be very ancient; perhaps original material from the birth of the Solar System.

Knowing ”how long is a day on Jupiter” just scratches the surface of the intrigue that is the Jovian system. You could spend months researching the planet and its moons, yet have more to research to do.

Here’s an article on Universe Today that shows how Jupiter can be very flattened, and an article about how the powerful windstorms are generated from its rotation.

NASA’s Ask an Astronomer also has an answer for the question, “how long is a day on Jupiter?” And a cool video of Jupiter’s rotation.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.

Sources:
NASA
Caltech Cool Cosmos

June 16, 2008December 24, 2015 by Nancy Atkinson

New Radio Telescope to Help SETI Scan Unexplored Frequencies for Extraterrestrials

Since the 1960’s astronomers have been scanning the heavens, searching for radio signals beamed towards the vicinity of Earth by other intelligent beings. But so far, no ET signals have been found. However, no radio telescope has been able to search the very low frequency radio spectrum, which could possibly include “leakage” of extraterrestrial “everyday” signals that a distant civilization might emit, such as television and radio signals. But a new radio telescope called LOFAR (the Low Frequency Array), will have that ability. Currently being built by ASTRON, (the Netherlands Foundation for Research in Astronomy), LOFAR consists of about 25,000 small antennas that will receive signals from space, and offers the ability to search these low-frequency type of radio waves.

According to Professor Michael Garrett, General Director of ASTRON, LOFAR is well suited to SETI research. “LOFAR can extend the search for extra-terrestrial intelligence to an entirely unexplored part of the low-frequency radio spectrum, an area that is heavily used for civil and military communications here on Earth. In addition, LOFAR can survey large areas of the sky simultaneously – an important advantage if SETI signals are rare or transient in nature.”

Astronomers believe of the approximately 100 thousand million stars in the galaxy, most of these have planetary systems. Some of these planets might actually be suitable for life and many scientists believe that life is probably wide-spread across the galaxy. However, technically advanced civilizations might be relatively rare or at least widely separated from each other.

Despite the huge distances between stars, the next generation of radio telescopes, such as LOFAR, begin to offer the possibility of detecting radio signals associated with extraterrestrial radio and TV transmitters.

Dan Werthimer, a SETI@home project Scientist at the University of Berkeley said, “SETI searches are still only scratching the surface, we need to use as many different telescopes, techniques and strategies as possible, in order to maximize our chances of success.”

Professor Garrett thinks it is high time European scientists began to support their colleagues from the United States in this exciting area of research. “I cannot think of a more important question humanity can ask and perhaps now answer. Are we truly alone in the Universe or are there other civilizations out there waiting to be discovered? Either way, the implications are tremendous.”

LOFAR will begin its scans of low frequency radio waves when the array is completed in 2009.

Original News Source: ASTRON

June 16, 2008December 24, 2015 by Nancy Atkinson

Three “Super-Earths” Found Orbiting One Star

Artist's impression of the trio of super earths. Image credit: ESO

“Does every single star harbor planets and, if yes, how many?” wonders planet hunter Michel Mayor. “We may not yet know the answer but we are making huge progress towards it.” Mayor and his team of European astronomers have found a star which is orbited by at least three planets. Using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the ESO La Silla Observatory, they have found a triple system of super-Earths around the star HD 40307. This is the first system known to have at least three “super-Earth” sized planets.

Back in 1995, Mayor, along with Didier Queloz, made the first discovery of an extrasolar planet around 51 Pegasi, and since then more than 270 exoplanets have been found, mostly around sun-like stars.

Most of these planets are giants, such as Jupiter or Saturn, and current statistics show that about 1 out of 14 stars harbors this kind of planet.

“With the advent of much more precise instruments such as the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, we can now discover smaller planets, with masses between 2 and 10 times the Earth’s mass,” says StÃ©phane Udry, one of Mayor’s colleagues. Such planets are called super-Earths, as they are more massive than the Earth but less massive than Uranus and Neptune (about 15 Earth masses).

HD 40307 is slightly less massive than our Sun, and is located 42 light-years away towards the southern Doradus and Pictor constellations.

“We have made very precise measurements of the velocity of the star HD 40307 over the last five years, which clearly reveal the presence of three planets,” says Mayor.

The planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively.

The group made the announcement at a conference about extrasolar planets being held in France. The same team also announced the discovery of two other planetary systems, also with the HARPS spectrograph. In one, a super-Earth (7.5 Earth masses) orbits the star HD 181433 in 9.5 days. This star also hosts a Jupiter-like planet with a period close to 3 years. The second system contains a 22 Earth-mass planet having a period of 4 days and a Saturn-like planet with a 3-year period as well.

“Clearly these planets are only the tip of the iceberg,” says Mayor. “The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days.”

A planet in a tight, short-period orbit is indeed easier to find than one in a wide, long-period orbit.
“It is most probable that there are many other planets present: not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet. Add to it the Jupiter-like planets already known, and you may well arrive at the conclusion that planets are ubiquitous,” concludes Udry.

Calculations from the sample of stars studied with HARPS implies that one solar-like star out of three harbors planets with masses below 30 Earth masses and an orbital period shorter than 50 days.

News Source: ESO press release

June 16, 2008December 24, 2015 by Ian O'Neill

Rare Asteroid Studied by Hawaiian Scientists

A huge impact with the asteroid Vista created a lot of debris. Credit :Don Davis)

Asteroid 10537 (1991 RY16) is a rarity. It is composed of basaltic rock (i.e. rock that cooled quickly after formation from a molten state) and appears to have evolved independently from the large asteroid Vesta. Vesta suffered a huge impact billions of years ago, and the debris from this collision litters the inner asteroid belt. These “Vestoids” make up the majority of the basaltic asteroids apart from three known isolated bodies including asteroid 1991 RY16. Scientists are therefore very interested to understand the evolution of 1991 RY16, possibly helping us understand the formation of the Solar System and why there aren’t more basaltic asteroids out there…

The asteroid belt occupies the volume of space roughly between the orbits of Mars and Jupiter. There are thousands of known rocky bodies in the belt, but half of the mass can be found in four major asteroids; Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. Ceres is actually classified as a minor (or dwarf-) planet as it is over 900km (560 miles) in diameter and is roughly spherical, unlike other asteroids that are irregular in shape. Large asteroid Vesta suffered a huge impact during the formation of the Solar System some 3.5 billion years ago and the debris (about 1% of its total mass) from this collision can be found scattered around the orbit of Vesta (~2.4 AU). These Vestoids usually explain many of the basaltic asteroids in this region of the asteroid belt.

So where does 1991 RY16 come in? Researchers at the Institute for Astronomy (IfA), University of Hawaii, carried out an analysis of the object after a previous study that utilized the Sloan Digital Sky Survey Moving Object Catalog. The IfA astronomers then used optical and near-infrared observations to derive spectroscopic data for 1991 RY16 to see whether it can be related to any of the asteroid groups in the asteroid belt. It turns out that its basaltic surface composition doesn’t appear to match up with any of the large groups of asteroids, and if its orbital radius is worked into the equation, it is highly unlikely that it could have travelled from any of the groups. 1991 RY16 appears to be an asteroid loner… or does it?

Firstly, the 5-15 km wide asteroid had to be ruled out from being a more common Vestoid. For a start 1991 RY16 isn’t even a remotely close spectroscopic match to any of the known Vestoids. Its orbit beyond the 3:1 Jupiter orbital resonance (at a distance of 2.5 AU) suggests that it could not have travelled from 2.4 AU, through the resonance and to its present orbit of 2.85 AU. The orbital resonances of the larger planets cause separation in the asteroid belt populations, confining them to their orbits. So, 1991 RY16 doesn’t originate from the Vesta impact event 3.5 billion years ago. Looking at the positions of the known asteroids (chart pictured), the IfA group ruled out the association of 1991 RY16 with any of the neighbouring asteroid groups (such as Gefion and Eos) as there is little spectroscopic evidence and it isn’t possible that the asteroid simply drifted (even after considering the strange Yarkovsky effect that predicts small rocky bodies experience a small deflection in trajectory due to anisotropic emission of thermal photons).

The possible remaining explanation could lie with a large asteroid near the orbital vicinity of 1991 RY16. The spectroscopic analysis of 1991 RY16 reveals that it could be a large chunk from another, differentiated asteroid. Although more analysis is required, 349 Dembowska (of ~140km in diameter) could be the parent asteroid 1991 RY16 was chipped from during an impact in the young Solar System. The IfA researchers are keen to point out that more observations are required to see if there is any other debris from this possible collision matching the surface composition of 1991 RY16.

For more detail into this very interesting research, check out the paper below.

Source: “A Spectroscopically Unique Main Belt Asteroid: 10537 (1991 RY16)” (arXiv pdf)

June 14, 2008December 24, 2015 by Nancy Atkinson

STS-124: A Mission in Pictures

Always a beautiful sight, the space shuttle Discovery touched down safely at 11:15 a.m. EDT, on Saturday, June 14, 2008, at the Kennedy Space Center in Florida. During the 13-day mission, Discovery and the crew of STS-124 delivered the new component Kibo, the Japanese Experiment Module, to the International Space Station. Mission managers say Discovery looks to be in good shape following the mission, and the crew is doing well, too. Even Garrett Reisman, who spent over 90 days on the ISS, joined the rest of the crew in walking around on the runway and surveying the shuttle. After a successful mission, its always fun to look back at some of the great images, so here’s a few…

Astronaut Ron Garan, STS-124 mission specialist, participates in the mission’s first EVA to get ready to add the Kibo Japanese Pressurized Module to the space station.

That’s two domes and two space helmets. Mark Kelly (right), STS-124 commander, and Garrett Reisman, assist astronauts Mike Fossum (left) and Ron Garan in the Quest Airlock of the International Space Station to help them get ready for an EVA.

A good look at two of the ISS solar arrays, which provide power to the station.

The ISS keeps growing, and with the addition of the Kibo lab, its actually getting pretty spacious on board the station.

The crew of the ISS took this image of the shuttle as it departed from the station, showing the now empty payload bay.

And likewise, the shuttle crew took this image of the ISS, showing the new configuration with Kibo now part of the station.

And here’s where it all started: the launch of Discovery on May 31, 2008.

See all the images from the mission here.

June 14, 2008October 6, 2016 by Matt Williams

What is the Diameter of Earth?

Our beautiful, precious, life-supporting Earth as seen on July 6, 2015 from a distance of one million miles by a NASA scientific camera aboard the Deep Space Climate Observatory spacecraft. Credits: NASA

For those people who have had the privilege of jet-setting or traveling the globe, its pretty obvious that the world is a pretty big place. When you consider how long it took for human beings to settle every corner of it (~85,000 years, give or take a decade) and how long it took us to explored and map it all out, terms like “small world” cease to have any meaning.

But to complicate matters a little, the diameter of Earth – i.e. how big it is from one end to the other – varies depending on where you are measuring from. Since the Earth is not a perfect sphere, it has a different diameter when measured around the equator than it does when measured from the poles. So what is the Earth’s diameter, measured one way and then the other?

Oblate Spheroid:

Thanks to improvements made in the field of astronomy by the 17th and 18th centuries – as well as geodesy, a branch of mathematics dealing with the measurement of the Earth – scientists have learned that the Earth is not a perfect sphere. In truth, it is what is known as an “oblate spheroid”, which is a sphere that experiences flattening at the poles.

*Data from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color. Credit: Geodesy2000*

According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator.

Equatorial vs Polar Diameter:

Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. As a result, the latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi).

In short, objects located along the equator are about 21 km further away from the center of the Earth (geocenter) than objects located at the poles. Naturally, there are some deviations in the local topography where objects located away from the equator are closer or father away from the center of the Earth than others in the same region.

The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively.

Meanwhile, the highest point on Earth is Mt. Chiborazo. The peak of this mountain reaches an attitude of 6,263.47 meters (20,549.54 ft) above sea level. But because it is located just 1° and 28 minutes south of the equator (at the highest point of the planet’s bulge), it receives a natural boost of about 21 km.

Mean Diameter:

Because of the discrepancy between Earth’s polar and equatorial diameter, astronomers and scientists often employ averages. This is what is known as its “mean diameter”, which in Earth’s case is the sum of its polar and equatorial diameters, which is then divided in half. From this, we get a mean diameter of 12,742 km (7917.5 mi).

The difference in Earth’s diameter has often been important when it comes to planning space launches, the orbits of satellites, and when circumnavigating the globe. Given that it takes less time to pass over the Arctic or Antarctica than it does to swing around the equator, sometimes this is the preferred path.

We have written many interesting articles about the Earth and mountains here at Universe Today. Here’s Planet Earth, The Rotation of the Earth, What is the Highest Point on Earth?, and Mountains: How Are They Formed?

Here’s how the diameter of the Earth was first measured, thousands of years ago. And here’s NASA’s Earth Observatory.

We did an episode of Astronomy Cast just on the Earth. Give it a listen, Episode 51: Earth.

Sources:

June 13, 2008December 24, 2015 by Nancy Atkinson

Latest Phoenix Images: Ice or Salt?

The Phoenix lander team revealed the latest images from the mission at a press briefing on Friday. This first image shows an area dug by Phoenix’s scoop, which disclosed a bright surface just a few inches down, which may be ice. “There’s still some debate about the bright material,” said Phoenix Principle Investigator Peter Smith. “Not everyone is sure that this is ice. So there’s been some debate on our team, centering around that perhaps there’s a salt layer just under the soil that also would be bright. Everyone does believe there’s ice under the surface, and whether this is ice or not is the question. The other question is, is this thick ice that goes down deep beneath the surface, or is this a thin layer and we’ll be able to scrape through? So being able to scrape with our scoop is a high priority for us.”

This pair of images taken by the Optical Microscope on NASA’s Phoenix Mars Lander offers a side-by-side comparison of an airfall dust sample collected on a substrate exposed during landing (left) and a soil sample scooped up from the surface of the ground beside the lander. In both cases the sample is collected on a silicone substrate, which provides a sticky surface holding sample particles for observation by the microscope.

Similar fine particles at the resolution limit of the microscope are seen in both samples, indicating that the soil has formed from settling of dust.

The microscope took the image on the left during Phoenix’s Sol 9 (June 3, 2008), or the ninth Martian day after landing. It took the image on the right during Sol 17 (June 11, 2008).

The scale bar is 1 millimeter (0.04 inch).

This is the latest color image of Phoenix, its surroundings and the scoop with soil.

While we can’t look inside the Thermal and Evolved Gas Analyzer (TEGA) oven which will “bake” the Martian soil to test the type of gases that are released, we can see that some of the soil has gone into TEGA. “We were finally successful and some of the material has slid down over the screen” said Smith, “sort of like material going over a cheese grater, and some of the material has slid down and filled the oven. We sent the commands for the first operation of TEGA last night, but we don’t have our data back yet, so we can’t report on any results. That will be coming later next week. So this is a very exciting time for us. We find the soil is very clumpy, it’s sticky, it’s an unusual soil not at all like the types of soils we used in our tests, which worked just fine with all the instruments. So we’ve developed another method of collecting samples, which is to tilt the scoop and vibrate it, and so it shakes down a small amount of material onto the instruments.”

And finally, here’s the latest weather report for Mars, on the 17th sol of Phoenix’s stay on Mars.

Sources: Phoenix News, NASA TV

June 13, 2008December 24, 2015 by Fraser Cain

How Long is a Year on Earth?

The eccentricity in Mars' orbit means that it is . Credit: NASA

A year on Earth is obviously 1 year long, since it’s the standard of measurement. But we can break it down further.

A year is 365.24 days. Or 8,765 hours, or 526,000 minutes, or 31.6 million seconds.

The tricky one is the number of days. Because the earth year doesn’t work out to exactly 365 days, we have the leap year. If we didn’t, days in the calendar wouldn’t match up with the position of the Earth in its orbit. Eventually, the months would flip around, and the northern hemisphere would have summer in January, and vice versa.

To fix this, we put on extra days in some years, called leap years. In those leap years, a year lasts 366 days, and not the usual 365. This gets tacked onto the end of February. Normally, February only has 28 days, but in leap years, it has 29 days.

When to you have leap years? It’s actually pretty complicated.

The basic rule is that you have a leap year if you can divide the year by 4. So 2004, 2008, etc. But years divisible by 100 are not leap years. So 1800, 1900 aren’t leap years. Unless they’re divisible by 400. So 1600 and 2000 are leap years. By following this algorithm, you can have an Earth orbit that lasts 365.24 days.

With the current system, it’s not actually perfect. There’s an extra 0.000125 days being accumulated. Over course of 8,000 years, the calendar will lose a single day.

Here’s an article about how astronomers might use cosmic rays to measure time on Earth.

And here is more information on how to calculate leap years from timeanddate.com.

We did an episode of Astronomy Cast just on the Earth. Give it a listen, Episode 51: Earth.