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April 25, 2017 2:15 PM Subscribe
Every day, our planet rotates 360°, right? Only if you mean a Sidereal Day. Solar days are 4 minutes longer on average.
Since Earth is also revolving around the sun, our planet needs to spin more than 360° between two successive noons.
If the two days were the same length, then the sky would look the same throughout the year. The discrepancy between solar and sidereal days means that the portion of the sky visible at night shifts by ~1 degree each night. Therefore, the night sky in March looks very different from the night sky in September.
Stars spend half the year (on average) rising and setting during daylight, until they finally appear just moments before daybreak. The night that a star transitions from being trapped in sunlight to finally visible is said to be its Dawn Rising.
Since Earth is also revolving around the sun, our planet needs to spin more than 360° between two successive noons.
If the two days were the same length, then the sky would look the same throughout the year. The discrepancy between solar and sidereal days means that the portion of the sky visible at night shifts by ~1 degree each night. Therefore, the night sky in March looks very different from the night sky in September.
Stars spend half the year (on average) rising and setting during daylight, until they finally appear just moments before daybreak. The night that a star transitions from being trapped in sunlight to finally visible is said to be its Dawn Rising.
Would you people stop making me feel dizzy today?
Cool post, the geometry and mathematics of the simple daily rhythms are endlessly fascinating. The history of discovering them even more so.
posted by Abehammerb Lincoln at 2:42 PM on April 25, 2017
Cool post, the geometry and mathematics of the simple daily rhythms are endlessly fascinating. The history of discovering them even more so.
posted by Abehammerb Lincoln at 2:42 PM on April 25, 2017
This topic is kind of a hobby horse of mine!
Only if you mean a Sidereal Day. Solar days are 4 minutes longer on average
It might be clearer to say that the sidereal day is 4 minutes shorter, since the solar day is the standard length of time that we divide up into 24 hours or 1440 minutes.
And of course the same exact thing happens with the moon revolutions, which we gauge relative to the sun, so we see new moons less often than the moon's actual orbital period.
posted by aubilenon at 2:51 PM on April 25, 2017 [2 favorites]
Only if you mean a Sidereal Day. Solar days are 4 minutes longer on average
It might be clearer to say that the sidereal day is 4 minutes shorter, since the solar day is the standard length of time that we divide up into 24 hours or 1440 minutes.
And of course the same exact thing happens with the moon revolutions, which we gauge relative to the sun, so we see new moons less often than the moon's actual orbital period.
posted by aubilenon at 2:51 PM on April 25, 2017 [2 favorites]
Would someone please take a one photo just before dawn in the Atacama Desert, every day of the year, and then turn it into a time lapse so that I can watch the stars rotate out from under the daylight sky? It is all I ask.
posted by polecat at 2:56 PM on April 25, 2017 [9 favorites]
posted by polecat at 2:56 PM on April 25, 2017 [9 favorites]
Which is to say that I searched for "heliacal rising" in YouTube and only got a bunch of stoner metal videos and assorted woo
posted by polecat at 3:00 PM on April 25, 2017 [2 favorites]
posted by polecat at 3:00 PM on April 25, 2017 [2 favorites]
So if our Sidereal Day is 23 hours 56 mins (something something seconds) to go 360 degrees, but 24 hours to go 361 (?) degrees, what about other planets in our system? When reference pages say Mercury's day is 1,408 hours, are they talking about a solar or sidereal day, usually?
posted by TigerB at 3:04 PM on April 25, 2017 [1 favorite]
posted by TigerB at 3:04 PM on April 25, 2017 [1 favorite]
So, if Earth has 4 corner simultaneous 4 day TIME CUBE within single solar rotation, and sidereal days are FOUR minutes shorter, but TIME OF NOON between days is total of 361° THEN
WE CAN NOT IGNORE
SIMULTANEOUS SIDEREAL AND BELLY BUTTON DAYS
16 DAY TIME HYPERCUBE
posted by curiousgene at 3:08 PM on April 25, 2017 [22 favorites]
WE CAN NOT IGNORE
SIMULTANEOUS SIDEREAL AND BELLY BUTTON DAYS
16 DAY TIME HYPERCUBE
posted by curiousgene at 3:08 PM on April 25, 2017 [22 favorites]
Came for the Time Cube joke; was not disappointed.
posted by KingEdRa at 3:12 PM on April 25, 2017 [4 favorites]
posted by KingEdRa at 3:12 PM on April 25, 2017 [4 favorites]
TigerB: Usually sidereal. Note that the farther out you get in the solar system (or the faster your planet is rotating), the smaller the difference between the two. Conversely, the distinction is gigantic for Mercury, such that anything that mentions the day is likely to sit and talk about the fact.
posted by Four Ds at 3:30 PM on April 25, 2017
posted by Four Ds at 3:30 PM on April 25, 2017
Yeah, it twists your head, no?
I have accumulated some errata over the years--re-twisting of perceptions--that I have found to be of no use at all. I'm not absolutely sure it's all accurate.
The stars move forward in the sky a little ways each sunset, but the full moon lags a bit each night in its rising. If you spend enough nights outside you get a feel for it, though you have to lie still and let your mind slip out of gear to understand what your eyes tell you. Also, if you practice, you can lie on your back at night and watch the Earth move--this is especially obvious at dawn, if you tell yourself that the Earth is turning into the Sun. At midnight if you look straight up, you can see the direction the Earth is headed (at the time). Half the year you are looking ahead, half the year you are looking behind the orbital path around the sun.
When I was a kid I watched the cleats on a caterpillar tractor's tread stop moving when they hit the ground, while the cleats on the top of the tread kept moving. I still have trouble granting this image the veracity it requires. But watching the stem on a bicycle tire (as the bike moved along) gave me notions about the Moon's orbit about the Earth that I find unsettling. Does it really speed up and slow down? Does its actual path resemble a sine wave rather than (more or less) a circle?
Too many nights by the campfire watching the sky has sent me down a few rabbit holes from which I'm not too anxious to emerge.
posted by mule98J at 3:47 PM on April 25, 2017 [7 favorites]
I have accumulated some errata over the years--re-twisting of perceptions--that I have found to be of no use at all. I'm not absolutely sure it's all accurate.
The stars move forward in the sky a little ways each sunset, but the full moon lags a bit each night in its rising. If you spend enough nights outside you get a feel for it, though you have to lie still and let your mind slip out of gear to understand what your eyes tell you. Also, if you practice, you can lie on your back at night and watch the Earth move--this is especially obvious at dawn, if you tell yourself that the Earth is turning into the Sun. At midnight if you look straight up, you can see the direction the Earth is headed (at the time). Half the year you are looking ahead, half the year you are looking behind the orbital path around the sun.
When I was a kid I watched the cleats on a caterpillar tractor's tread stop moving when they hit the ground, while the cleats on the top of the tread kept moving. I still have trouble granting this image the veracity it requires. But watching the stem on a bicycle tire (as the bike moved along) gave me notions about the Moon's orbit about the Earth that I find unsettling. Does it really speed up and slow down? Does its actual path resemble a sine wave rather than (more or less) a circle?
Too many nights by the campfire watching the sky has sent me down a few rabbit holes from which I'm not too anxious to emerge.
posted by mule98J at 3:47 PM on April 25, 2017 [7 favorites]
24 hours / 365 = 3 minutes 56.71 seconds
Intuition pump: If people on the Moon counted days using the Earth rather than the sun, their days would be infinitely long.
posted by a car full of lions at 4:14 PM on April 25, 2017 [1 favorite]
Intuition pump: If people on the Moon counted days using the Earth rather than the sun, their days would be infinitely long.
posted by a car full of lions at 4:14 PM on April 25, 2017 [1 favorite]
In college, I took a hybrid humanities-sciences course about the place of science in society. One of our first assignments, which lasted weeks, was to track the time a star passed behind some immobile object like a flagpole or the roofline of a house. After a few weeks, we plotted our observations and... woah... what is this? We'd calculated the length of the sidereal day and I still think it was one of the coolest, slowest-building homework assignments of my life.
posted by late afternoon dreaming hotel at 4:27 PM on April 25, 2017 [5 favorites]
posted by late afternoon dreaming hotel at 4:27 PM on April 25, 2017 [5 favorites]
Days where I understand things that had vaguely bothered me for a while are good, good days. Even with this fucking virus that won't leave.
posted by ROU_Xenophobe at 4:41 PM on April 25, 2017 [1 favorite]
posted by ROU_Xenophobe at 4:41 PM on April 25, 2017 [1 favorite]
I never understood sidereal time until I read your first link. Thank you!
posted by Johnny Wallflower at 5:18 PM on April 25, 2017
posted by Johnny Wallflower at 5:18 PM on April 25, 2017
"At midnight if you look straight up, you can see the direction the Earth is headed (at the time)."
That doesn't seem right... At midnight the direction the earth is heading should be more or less your eastern horizon.
posted by xris at 5:37 PM on April 25, 2017 [2 favorites]
That doesn't seem right... At midnight the direction the earth is heading should be more or less your eastern horizon.
posted by xris at 5:37 PM on April 25, 2017 [2 favorites]
Another fun thought that takes a while to grock:
In the winter the northern hemisphere (NH) is tilted -away- from the Sun. As a result, during the day, the Sun's low on the southern horizon. But at nighttime, the NH is tipped -toward- the stars in the southern skies. Thus, for example, Orion's high in the sky.
This also means that the ecliptic ... along which the Moon and planets travel (roughly) ... is high in the sky on winter nights. Not only that ... take a look at an astronomy program (like Stellarium), speed up the action, and watch the ecliptic dance around the horizon. No damned wonder the ancients complained about how the Moon wanders all over the sky.
posted by Twang at 5:37 PM on April 25, 2017 [1 favorite]
In the winter the northern hemisphere (NH) is tilted -away- from the Sun. As a result, during the day, the Sun's low on the southern horizon. But at nighttime, the NH is tipped -toward- the stars in the southern skies. Thus, for example, Orion's high in the sky.
This also means that the ecliptic ... along which the Moon and planets travel (roughly) ... is high in the sky on winter nights. Not only that ... take a look at an astronomy program (like Stellarium), speed up the action, and watch the ecliptic dance around the horizon. No damned wonder the ancients complained about how the Moon wanders all over the sky.
posted by Twang at 5:37 PM on April 25, 2017 [1 favorite]
I recall having this thought when I was younger: If the earth is about 24,000 miles around and a day (one rotation) is 24 hours, that means it is spinning 1,000 miles an hour! Why don't I go flying off?
Realized after astronomy and chemistry that many of the forces holding things together...and what happens when we take them apart/speed 'em up/slow 'em down...are beyond my ken; solar systems are just like atoms, etc.
posted by CrowGoat at 6:39 PM on April 25, 2017
Realized after astronomy and chemistry that many of the forces holding things together...and what happens when we take them apart/speed 'em up/slow 'em down...are beyond my ken; solar systems are just like atoms, etc.
posted by CrowGoat at 6:39 PM on April 25, 2017
At midnight the direction the earth is heading should be more or less your eastern horizon
Agreed. If you look straight up at midnight your back is facing the sun. The tangent to Earth's orbit is to the east. We know we're moving east because any given star is higher in the sky at dusk as the year progresses -- the star appears to us to be moving toward the west.
posted by polecat at 8:04 PM on April 25, 2017 [1 favorite]
Agreed. If you look straight up at midnight your back is facing the sun. The tangent to Earth's orbit is to the east. We know we're moving east because any given star is higher in the sky at dusk as the year progresses -- the star appears to us to be moving toward the west.
posted by polecat at 8:04 PM on April 25, 2017 [1 favorite]
Conversely, the distinction is gigantic for Mercury, such that anything that mentions the day is likely to sit and talk about the fact.
Mercury rotates 3 times for every 2 orbits. Rotating once per orbit means you're tide locked (and you don't even have days and nights), so that means every 2 of their years they have a day. That's pretty weird.
posted by aubilenon at 8:18 PM on April 25, 2017
Mercury rotates 3 times for every 2 orbits. Rotating once per orbit means you're tide locked (and you don't even have days and nights), so that means every 2 of their years they have a day. That's pretty weird.
posted by aubilenon at 8:18 PM on April 25, 2017
Does it really speed up and slow down?
Yes, and No. These things are all relative, as somebody famous once said.
Does its actual path resemble a sine wave rather than (more or less) a circle?
Yep.
posted by HiroProtagonist at 10:15 PM on April 25, 2017
Yes, and No. These things are all relative, as somebody famous once said.
Does its actual path resemble a sine wave rather than (more or less) a circle?
Yep.
posted by HiroProtagonist at 10:15 PM on April 25, 2017
The solar component of Hindu components is based on the sidereal, rather than the tropical year. This is why Hindu astrology has diverged from Western astrology over a period of many centuries, despite sharing some common origins - the equivalent zodiac signs differ by a few minutes every year, which adds up to a lot over time.
(Some Hindu calendars are solar, and some are lunisolar, but the solar component is always sidereal).
The Indian calendars.
In another part of the world, the the Dog Days of summer are named after the rising of the star Sirius, which the ancient Egyptians linked with the flooding of the Nile. Nowadays Sirius is a winter star, due to the precession of the equinoxes and the difference between the sidereal and tropical year. Things change.
posted by plep at 11:59 PM on April 25, 2017 [1 favorite]
(Some Hindu calendars are solar, and some are lunisolar, but the solar component is always sidereal).
The Indian calendars.
In another part of the world, the the Dog Days of summer are named after the rising of the star Sirius, which the ancient Egyptians linked with the flooding of the Nile. Nowadays Sirius is a winter star, due to the precession of the equinoxes and the difference between the sidereal and tropical year. Things change.
posted by plep at 11:59 PM on April 25, 2017 [1 favorite]
Every day, our planet rotates 360°, right? Only if you mean a Sidereal Day. Solar days are 4 minutes longer on average.
Although it does feel like the world is spinning much faster these days.
posted by sour cream at 12:16 AM on April 26, 2017
Although it does feel like the world is spinning much faster these days.
posted by sour cream at 12:16 AM on April 26, 2017
I made an animation about this! What a fun job.
posted by michaelhoney at 4:02 AM on April 26, 2017 [4 favorites]
posted by michaelhoney at 4:02 AM on April 26, 2017 [4 favorites]
That doesn't seem right... At midnight the direction the earth is heading should be more or less your eastern horizon.
You are right. I guess I need to toss another limb on the fire.
posted by mule98J at 2:52 PM on April 26, 2017 [1 favorite]
You are right. I guess I need to toss another limb on the fire.
posted by mule98J at 2:52 PM on April 26, 2017 [1 favorite]
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posted by Chrysostom at 2:40 PM on April 25, 2017