For thousands of years, cultures have used celestial bodies to track the passage of time. Few details about timekeeping are available about prehistoric areas, but it's believed that the first known calendar to measure time was made by the Sumerians just 5,000 years ago. Over the centuries, various cultures — from the Egyptians to the Aztecs to the Chinese — have divided their calendars into days, weeks, and months based on the movements of the sun, moon, and Earth, which brings us to the main difference between the lunar and solar calendars.

Much like the difference between solar and lunar years, a lunar calendar is based on the cycle of the moon, while a solar calendar is based on the position of the sun and corresponding seasons as Earth orbits its star. As a result, one month on a lunar calendar doesn't begin or end at the same time as the same month on a solar calendar. On top of that, there's a lunisolar calendar that synchronizes both types of timekeeping, although not perfectly. These differences are better to understand after learning about how each of these calendars works.

The Sumerians are likely the first people to base a calendar on the different phases of the moon. As the older of the two calendars, the lunar calendar is easier to follow because all you have to do is observe each phase of the moon's cycle. It doesn't measure the passage of time down to a tee, though.

One lunar month (also called a synodic month) starts on the day of the new moon and ends on the following new moon — about 29.5 days. With 12 months in a year, which results in 354 days, there is an average of 11 days unaccounted for compared to a solar year. Buddhist and Hebrew cultures compensate for that by adding a 13th month every two or three years to resynchronize the lunar calendar with the seasons. Since the Islamic Hijri calendar doesn't compensate for the time gap, though, important cultural events occur earlier each year. Even the seasons begin at slightly different times.

The first day of the lunar month varies from culture to culture as well. Generally, each synodic month starts either with a new moon or with the first visible crescent moon a day or so after the new moon. Yet, the month on some Hindu lunar calendars begins the day after a full moon.

While the Egyptians initially used a calendar based on the phases of the moon, they switched around 3100 B.C. to tracking the passage of time by watching for the Sirius star in Canis Major — a constellation near Orion, to the lower left — to rise next to the sun. Their commitment to counting the 365 days between the star rising in the same position led to the discovery that it takes that long for Earth to orbit the sun. It's one of the oldest records of a solar calendar — featuring 12 months, each 30 days long, and five extra days to celebrate the gods. Since Earth's orbit is actually 365.25 days long, the Egyptian calendar length was adjusted to include an extra day every four years (leap year) in 237 B.C.

The Julian calendar was established in 45 B.C. to include one month (February) of 28 days and 11 months of 30 or 31 days each, taking into account the leap year with an extra day in the shortest month every four years. By the mid-1500s, though, there was an excess of 10 days, so the Gregorian calendar was created in 1582 to correct the timekeeping issue. While it has the same number of months and number of days in each month as the Julian calendar, it omits leap years on centurial years indivisible by 400 and accounts for the passage of the seasons based on the sun's position in Earth's sky. Most of the world still uses this solar calendar, and the U.S. colonies adopted it in 1752.

In between all the updates and switching of calendars based on specific celestial bodies, some cultures around the world used a combination of lunar and solar calendars. This merged version is called a lunisolar calendars, and historians believe it was formulated sometime between 3000 and 2001 B.C. Ancient civilizations throughout Greece and the Middle East (except Egypt) used it to track both the moon phases and how the seasons change based on the Earth's orbit around the sun.

Generally, the months are based on the lunar cycles, while the year itself is based on Earth's orbit around the sun. One example is the Jewish calendar, which features 12 synodic months. Each month has 29 or 30 days, except for Ḥeshvan and Kislev, which each may have 29 or 30 days. As a result, a lunar year can be 353 to 355 days. Since that's shorter than a solar year, a 30-day month (First Adar) is added before Second Adar during a leap year, which then has 383 to 385 days. There are seven leap years every 19 years (235 synodic months), after which the lunar cycles fall on the same days in the solar year again.

The Chinese calendar is another lunisolar example with 12 months that are presented as numbers or Chinese zodiac animals. The months alternate in lengths of 29 and 30 days for a total of 354 days, so an extra month is added in leap years — which happens seven times within a 19-year period like the Jewish calendar — and takes the name of the previous month.

The explosion of the sun is a threat straight out of science fiction, but could it really happen, and if so, what would the consequences be for our solar system? When a star explodes, it forms a supernova, a sudden and massive burst of light that can outshine a whole galaxy. Supernovae leave their mark on the skies as nebulae, clouds of solar debris that make for many of the most iconic astronomical images ever captured. Supernovae can give birth to black holes and neutron stars, and their light can be observed from thousands of light years away.

Supernovae usually happen when a star stops producing nuclear fuel, causing a pressure imbalance between the core and the surface that makes the star collapse on itself. The force of this collapse generates shock waves that obliterate the star in a violent burst. If the sun were to explode in such a fashion, it would obviously be bad news for Earth and every other object in the solar system. The explosion would destroy the inner planets, but we humans wouldn't even make it to that point. Roughly 99% of the energy generated from the supernova would be spewed out in the form of neutrinos, which would vaporize everything on Earth before the shockwaves of the explosion even reach the planet. Fortunately, we'll never have to worry about such an event because the sun is not going to explode. Not now, not later, not ever. However, our sun will eventually die, but in a far different and much slower fashion.

Our sun will never go supernova because it simply isn't big enough. It would need to be about ten times more massive to collapse with such force. The life cycle of a star depends on its size, and the ultimate fate of a star the same mass as our sun is to essentially burn out. Before that happens, though, the heat is going to go way up.

The sun is primarily composed of hydrogen atoms, which collide under tremendous pressure and temperature with each other to form helium. This process, known as nuclear fusion, is how the sun generates energy, and that energy output is steadily increasing, albeit at a pace far too slow for us to notice. Scientists estimate that the sun's energy output has increased by roughly a third since its formation 4.6 billion years ago. That trend is expected to continue, with the sun burning about ten percent brighter every billion years.

Such an increase in solar energy will introduce Earth to a whole new kind of global warming. The polar ice caps will melt and the atmosphere will fill with water vapor that traps even more heat. By one or two billion years from now, temperatures will have risen so much that the oceans will boil away, leaving the planet as a lifeless wasteland, similar to present-day Venus. After the oceans evaporate, the atmosphere will dissipate, and ultimately, Earth will be left as a barren shell of its former self. But that's only the beginning of the end for the sun itself.

The sun has a limited supply of hydrogen which it steadily depletes through the process of nuclear fusion. At this point, NASA estimates that the sun is a little more than 70% hydrogen. The vast majority of what remains is helium, which hydrogen is converted into during nuclear fusion. Eventually, the sun will turn all of the hydrogen in its core into helium, and it will cease to generate energy. This point marks the end of a star's primary life sequence, but it isn't expected to happen for five billion years, meaning that the sun today isn't even halfway through its active lifespan.

The sun, like all stars, exists in a delicate balance between the outward pressure of its core and the inward pressure of gravity. When the core is depleted of hydrogen, it can no longer stand up to the force of gravity, causing the star to collapse. This is how supernovae are generated, but for a smaller star like the sun, the effect will be a bit less dramatic. The solar core will be compacted to an incredibly dense, hot state. At this point, the helium will begin fusing into heavier elements along with whatever traces of hydrogen remain in the outer layers of the sun. The heat will cause the star to expand, setting it on track to become a red giant.

The sun as we know it is an example of a main sequence star, the main sequence being the hydrogen-burning phase of a star's life. The sun's main sequence is estimated to last 10 billion years in total, ending around 5 billion years from now. After that, the sun's outer layers will expand, and it will become brighter. However, the surface temperature will cool by about half, which will change the sun's light from bright white to red. At this point, it will become a red giant, like the star Betelgeuse, which is commonly used to locate the constellation Orion.

It is unclear how big the sun will get when it enters the red giant phase. The red giant stars that have been observed so far range from 100 to 1,000 larger than the sun is today. The expansion would consume Mercury and Venus, and most likely Earth as well. If our planet did survive, it would be so close to the expanded sun that everything on the surface would incinerate. But the sun's transformation into a red giant will only push its habitable zone further, and this could potentially make icy dwarf planets in the Kuiper Belt, such as Eris, into oases of liquid water ready to harbor life. Unfortunately for any new life that might emerge at that point, the solar system won't have very much time left.

The red giant phase of a star only lasts about a billion years at best. While that could potentially be enough time for new life to emerge in the distant reaches of the universe, the habitable zone will end when the sun moves into the next stage of its death. Eventually, all of the helium that was fused from the sun's original hydrogen supply will also be exhausted, and all fusion will cease. The star will eject its outer layers, creating a glowing cloud called a planetary nebula. In the center of this nebula, the remaining core of the sun will contract into a white dwarf.

Once there is no fusion whatsoever in the sun's core, there will be nothing to resist gravity, causing the core to crush itself down to around the size of planet Earth. A white dwarf can reach temperatures approaching 200,000 degrees Fahrenheit, which is over 10 times the temperature of the sun's surface. However, that is significantly colder than the current core of the sun, which is about 27 million degrees. At this point, the sun is like the smoldering ashes left behind from a fire, all but dead.

With no more nuclear fusion occurring, a white dwarf cannot generate any energy. All that's left is for the residual heat left in the core to dissipate. If a white dwarf cools completely, to the point that it no longer emits any light or heat, it becomes a black dwarf, a dense clump composed primarily of carbon and oxygen, the products of helium fusion. The absence of light would make this corpse of a star invisible to the eye, and detectable only by the gravitational field it would maintain. However, all of this is merely theoretical, because not only has the sun not turned into a black dwarf yet, no other star in the Universe has.

White dwarfs take a long, long time to fully cool down. That's because they are so small and densely compacted that there is little surface area for heat to escape from. It would take trillions or potentially even quadrillions of years for a black dwarf to form. The Universe itself is only 13.8 billion years old today, which means that even the oldest stars are still in the early stages of the white dwarf phase. By the time the first black dwarf forms, our own sun will be further into the white dwarf phase than the oldest white dwarf in existence today.