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Dr James O'Donoghue Profile picture
@physicsJ
Sep 22, 2020 • 5 tweets • 2 min read • Read on X
It's Equinox in 1 hour! 22 Sep 2020, 13:30 UTC. Today everywhere has an almost** equal day and night length of 12 hours, while sunlight is at max intensity on the equator. Earth has seasons because of its axial tilt, which leads to day/night length changes throughout the year
**At equinox daylight always lasts OVER 12 hours, for everyone! But why?
1) Sunrise/set time is based on the edge of the Sun, not the middle.
2) Atmospheric refraction means light bounces around the Earth and shows up earlier than sunrise AND later than sunset
Both mean more 🌞! Image
Here are the southern hemisphere season names and values for 45° south latitude

And, if you want to find out what time equinox is for your location:
timeanddate.com/worldclock/fix…
It's Equinox *now* to the nearest minute
So there are links out there that'll say this occurred at 13:30 or 13:31

I got my value from NASA at data.giss.nasa.gov/modelE/ar5plot…. and also confirmed with timeanddate.com/calendar/septe…. The reason 13:31 UTC is out there is probably a difference in rounding up or down the seconds to minute

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More from @physicsJ

Dr James O'Donoghue Profile picture
Aug 11, 2022
Saturn will be at its closest point to Earth on Sunday. Ever seen it with your own eyes? Over the next couple of days Saturn will the brightest thing near the Moon. You can see it wherever you are on Earth, just look towards the south east, about an hour after sunset! 🪐 Image
So, Saturn is closest to the Moon tonight and Friday night (11-12 Aug), but Jupiter is closest from Saturday (13 Aug) onwards. To catch Jupiter, you'll have to wait a few hours after sunset. Scene should look something like this on Saturday evening. (All images via "Stellarium") Image
This is your view of Jupiter, Saturn and the Moon tonight, a few hours after sunset (northern hemisphere)! Image
Read 6 tweets
Dr James O'Donoghue Profile picture
Aug 10, 2022
Actual footage of stars orbiting the supermassive black hole Sag A* at the center of our galaxy. One of the stars, S0-2, moves faster than 27 million kilometers per hour, or 2.6% the speed of light!
Video source: and credit: Abhimat Gautam, Keck/UCLA Galactic Center Group
Sag A* HAS A MASS OF 4 MILLION SUNS! That's enough mass to allow me to use caps lock unquestionably. It's 26,000 light years away, so you're quite safe. Here's a computer model which shows the full orbits, based on the observations! Via galacticcenter.astro.ucla.edu/animations.html
Read 5 tweets
Dr James O'Donoghue Profile picture
Jul 25, 2022
The observable universe is 93 billion light years wide and contains two trillion galaxies EACH containing countless billions of stars

The UNobservable universe beyond is at least 250 times wider, so its volume and galaxy content is 15 million times larger

Enjoy your Monday🥵
Because we understand the "shape" of the observable universe, we are able to extrapolate how big it should be in its entirety. "we" meaning cosmologists like @StartsWithABang who wrote about this in detail here. I recommend reading it over a drink! medium.com/starts-with-a-…
Every part of the fabric of the universe is expanding. Pick two close-together galaxies, they're moving apart over time. But pick two far-apart galaxies, since there's more fabric between them they move apart faster. But let's not stop there...
Read 7 tweets
Dr James O'Donoghue Profile picture
Jul 23, 2022
Stars are formed in dusty regions of space like the nebula below, so the process is normally difficult to see in visible light (eg. with Hubble). JWST uses infrared which allows us to actually see *through* cosmic dust, giving us (literally) a window into the birth of stars!
You've really got to check out the full image taken by JWST btw, which can't be seen on Twitter anywhere near the full resolution. Check it out: webbtelescope.org/contents/media…. The highest resolution version has 15 times more pixels than a 4k image, you can even see background galaxies! Image
You can also compare Hubble and JWST images here with a slider, thanks @Samuel_Gregson
-> webbcompare.com Image
Read 4 tweets
Dr James O'Donoghue Profile picture
Jul 7, 2022
The video stems from my first/original 'science communication' animation idea. I had 500 followers when it dropped in Dec 2018 and was used to getting 0 or 1 likes on my posts before it. Because of your +ve reaction, I carried on 💪, so thank you!
I wanted to bring this up: I made an error in the 2018 vid. Someone corrected me on the length of day for Venus so I re-made the vid immediately. It's a teachable moment for what scientists do, so I left it, but all future posts are deleted when even the tiniest error is spotted
The length of day for Earth, yeah it's 23h 56m relative to the stars, but about 24 relative to the Earth. Here's a graphical explainer because of my "show don't tell" philosophy
Read 6 tweets
Dr James O'Donoghue Profile picture
Dec 1, 2021
Did you know the planets don't technically orbit the Sun's center, instead, everything orbits our solar system center of mass, including the Sun! While our star holds 99.8% of the solar system's mass, Jupiter/Saturn hold most of the rest, so the Sun orbits them slightly (thread)
Planet Earth follows the sun as it does this, keeping the Sun-Earth distance stable. Here's what our orbit looks like for the next hundred years. Note also that Earth's orbit is almost circular, so I put a circle on to show you how close it is
I hope the notes worked in the first video, I try not to clutter it too much, but the middle stuff is to scale... the Sun is rarely in the dead-center of the solar system
Read 4 tweets

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