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Earth, the planet we all know and love, is a pretty cool place. After all, it’s the only place in the Universe where life exists (as far as we know). The astronomical properties of the Earth imprint themselves on our daily lives in many ways, from how we tell time to the days of the week to the temperatures throughout the year. People have been studying the shape and size of the Earth for centuries, and the way we do astronomy is heavily influenced by the restrictions the Earth puts on us.


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Coordinates & Timekeeping

What are the correct dates for the starts and ends of the zodiac signs? (Beginner)

This is really a question for an astrologer rather than an astronomer. I know almost nothing about how zodiac signs work since they aren’t really based on anything actually going on in the sky. They are supposedly based on what constellation the Sun is currently in at that time of year, but the time boundaries have been shifted dramatically because otherwise the different signs would vary drastically (from 7 to 45 days). So each sign is given exactly 365/12 = 30.4 days of the year, with the first sign (Aries) starting at the precise moment of the spring equinox. Because the spring equinox varies from year to year, the starting/ending dates can shift by a day in either direction, so it is likely that both you and your teacher are right and it might just depend on what year (or timezone) you are currently in.

When Eratosthenes was measuring the circumference of the Earth, how did he know it was noon in Alexandria? (Beginner)

Thanks for the question. Noon is a concept that is far older than clocks or time zones that is defined simply by being the time that the Sun is highest in the sky. This is easy to keep track of by just following the Sun’s position over time, so ancient cultures used this very frequently for timekeeping. It is only recently (in the late 1800s) that people standardized clock times across large areas (time zones), meaning that the time when the Sun is the highest is not necessarily noon anymore (for instance, where I live, it happens at about 12:18pm or 1:18pm during Daylight Savings Time). Hopefully this answers your question.

What is the celestial equator? Why do we measure coordinates based off of it? (Beginner)

The celestial equator is the projection of the Earth’s equator onto the sky. At the equator this is straight up, but as you move further from the equator to higher latitudes, the celestial equator gets lower in the sky. The latitude can be determined by seeing how low the celestial equator is, however it is more practical to measure latitude by finding the altitude of the celestial pole since there is a nice pole star in the northern hemisphere.

It would be difficult to set up an apparatus to measure coordinates relative to the ecliptic directly because the ecliptic move throughout the night/year. The celestial equator is always in the same place at a given latitude so telescopes tend to be set up along that axis. The other natural coordinate system is altitude/azimuth, which is defined relative to the ground and zenith.

Why are the days of the week in the order they are? (Intermediate)

This is an interesting question I hadn’t thought about before. I didn’t know anything more than you about this when you sent this to me, but here is what I have discovered with a bit of research (there are a lot of layers to this, so buckle up):

In ancient Roman astronomy, the Universe was thought to be centered around the Sun, with 7 “planets” moving through the sky against a stationary background of stars. These seven planets were usually listed in order of their speeds relative to the stars because that was presumed to correspond to their distance from the Earth (these included the Sun and the Moon because the astronomers thought everything orbited around the Earth. Outer planets are not included because they weren’t discovered until telescopes were invented). In order of slowest to fastest, these seven planets are:

  1. Saturn
  2. Jupiter
  3. Mars
  4. Sun
  5. Venus
  6. Mercury
  7. Moon

In ancient Roman astrology, each hour of the day belonged to a different planet. These hours were assigned in order of their speed through the sky (the order above). So starting at sunrise on a given day, each of the 12 hours of daytime and 12 hours of nighttime were given a corresponding planet, with the list looping back to the top every 7 hours. This meant that over the course of the 24 hour day, the list looped ~3.5 times (3 full times with 3 extra planets for hours 22, 23, and 24). Continuing on the next day, hours would be offset by 3 places in from the hours in the day before. This process loops around until it cycles through all of the planets starting a day off, and each day is named after the planet that started it.

This is a really confusing system, so here’s an example that corresponds to this table on Wikipedia (look at the table, it’s very helpful). Day 1 starts with the planet in position 1 (the slowest one), which is Saturn, so it is called Saturday. Over the course of Saturday, the whole cycle of planets loops 3 times (shown in alternating white and gray across the first row of the table) with 3 extra planets in hours 22, 23, and 24 corresponding to Saturn, Jupiter, and Mars. This means that by the beginning of Day 2, we are already at position 4 in the list of planets, which is the Sun, so it is called Sunday. This process of skipping 3 planets every day continues throughout the week, making Day 3 start with the Moon, Day 4 Mars, Day 5 Mercury, Day 6 Jupiter, and Day 7 Venus. In languages descended from Latin (like French or Spanish) this is how the days of the week are named (with the exception of Saturday which was renamed after the Sabbath and Sunday which was renamed after God).

The last layer is how English and other Germanic languages got their days of the week. Germanic tribes simply stole the Latin calendar and switched the names of the Roman words/gods for Germanic ones (except for Saturday for some reason). So that path of planet orbits to hours in a day to Roman gods to Germanic gods is how all of the days of the week got named and arranged how they did.

Hopefully that was understandable. I had a fun time learning about all of this and I would recommend reading the rest of that Wikipedia page I linked before for more information and also looking at the one for the names of the days of the week. If you have any more questions, let me know

How is the equation of time derived? (Advanced)

Do you have any explanation of why the motion along the equatorial plane is considered when deriving the Equation of Time for measuring the precise length of the day as opposed to the ecliptic plane? What is the relationship between the Equation of Time (red), the eccentricity of the Earth’s orbit (blue) and obliquity of the Earth’s rotational axis (green)?


This is a topic that gets very complicated very quickly, so first I’ll do my best to simplify it. The motion of objects along the celestial equator is a direct reflection of the rotation of the Earth. For objects outside our solar system, the only factor influencing their motions is the rotation of the Earth, and this process takes a very regular amount of time. The time it takes for a star to go once around in the sky is called a “sidereal day” and it is about 23h56m long. The motion traced by the celestial equator is a very steady metronome, so it makes sense to base things off of it. The “mean solar day” we use to define our normal clock time is the average amount of time it takes for the Sun to go all the way around the sky. It is slightly longer than the sidereal day because the Sun lags about 4m (or 24h/365.26) per day relative to the stars because of its motion over the course of the year.
The Sun’s motion, as you have pointed out, is more complicated. The Sun moves through the sky along the ecliptic and not along the celestial equator, and since the ecliptic isn’t parallel to the celestial equator, some of the Sun’s motion goes up and down in the sky rather than around in the sky. This up and down motion doesn’t do anything to change the length of the solar day (just the height of the Sun) so the more diagonal the motion of the Sun is, the less progress it makes along the celestial equator. This leads to the speeding up and slowing down seen in the green curve above.
We don’t define a day based on the ecliptic because that’s not how anything else in the sky outside our solar system moves. The Earth doesn’t rotate on that axis, so if we were to try to track time based on the ecliptic, we would have to make big adjustments in the motions of everything else in the sky that would lead to huge problems. For centuries, astronomers and navigators have kept time based on the stars rather than the Sun because their motion is so much more reliable.
The blue curve in the graph doesn’t depend on the projection of the Sun onto the ecliptic but rather with the orbital speed of the Earth. The Earth’s orbit is slightly elliptical, meaning that for the part of the year that the Earth is closer to the Sun, it moves faster than average, and vice versa. When the Earth covers a larger than normal distance in its orbit in a given day, the Sun moves through the sky faster than it does on average, causing the deviation seen in blue.
For a much more detailed treatment of these subjects, I refer you to this paper which derives the relationships from first principles using orbital dynamics. It gets very complicated, but if you really want a “satisfactory” explanation, then you have to wade through all the math.


Why do people think the Earth is flat? (Beginner)

Let me start out by saying that the Earth is not flat and no serious scientists for the past 2000 years have ever thought it was flat. Ancient Greeks first determined that the Earth was round just by looking at how ships and mountains looked on the horizon (among other things) and Eratosthenes even determined the circumference of the Earth to within about 15%. Even throughout the Middle Ages, most people knew that the Earth was round, including Christopher Columbus and everyone surrounding his voyage.

The only people that say that the Earth is flat is a very small (but loud) group of people mostly on the internet. The internet allows like-minded people to find each other and (if they choose to) seal themselves off from all outside information into their own echo chamber, so these types of fringe conspiracy theories have had better success in the past 20 years or so. Some of the flat earthers are “trolls” who knowingly spread false (but sometimes compelling and well thought out) information for fun just to cause chaos. These people prey on other people who are more vulnerable to misinformation or delusion, like people who are undereducated or people with mental disorders like schizophrenia. This mirrors a dynamic in many abusive relationships called “gaslighting” where someone in a position of power (the troll) tells a victim to trust them completely and not believe anyone else, including themselves.

So in short, people don’t actually think the Earth is flat and the whole movement is a malicious conspiracy against vulnerable people. None of the scientific arguments they make are correct (though some loosely based on truth) so I won’t go into them here, but I’m sure that if you go to a flat earther website they will tell you all of their arguments (just be careful not to get sucked in. Their arguments are designed to be predatory). Hopefully this helps and let me know if you have any more specific questions.

How many miles do you have to travel over the Earth’s surface before it curves 12 inches downwards? (Intermediate)

The curvature of the Earth is about 8 inches per mile *per mile* on average (the per mile squared means the number of inches away from the surface of the Earth “accelerates” with a larger number of miles, for a straight line). Plugging in some numbers with trial and error, I quickly estimate that your straight line will travel about 1.22~1.23 miles before it is 12 inches from the average curved surface of the Earth. 


Why is winter the darkest season? Why is the night longer? (Beginner)

The difference in the “brightness” of the seasons is due to the Sun following a different path through the sky during the summer and winter. This isn’t because the Sun is moving, though, it is because the Earth’s rotational axis is tilted compared to its orbit. As the Earth moves around the Sun over the course of the year, the North pole stays pointed in the same direction, which is about 23 degrees away from straight up in the solar system. That means that in the summer, the northern hemisphere is pointed 23 degrees further towards the Sun, and in the winter it is pointed 23 degrees further away from the Sun. When the northern hemisphere is pointed at the Sun, it remains visible in the sky for longer in the day as the Earth rotates around, making the days hotter, and vice versa for the winter. I would recommend taking a look at this website if you want some visuals.