A Tiny Planet in the Cosmos: APOD 20181208

2:53

2018-12-08

[public] 61.1K views, 536 likes, dislikes audio only

AlphaPhoenix

This video featured as an Astronomy Picture of the Day!

https://apod.nasa.gov/apod/ap181208.html

Official Explanation:

You can pack a lot of sky watching into 30 seconds on this tiny planet. Of course, the full spherical image timelapse video was recorded on planet Earth, from Grande Pines Observatory outside Pinehurst, North Carolina. It was shot in early September with a single camera and circular fisheye lens, digitally combining one 24-hour period with camera and lens pointed up with one taken with camera and lens pointed down. The resulting image data is processed and projected onto a flat frame centered on the nadir, the point directly below the camera. Watch as clouds pass, shadows creep, and the sky cycles from day to night when stars swirl around the horizon. Keep watching, though. In a second sequence the projected center is the south celestial pole, planet Earth's axis of rotation below the tiny planet horizon. Holding the stars fixed, the horizon itself rotates as the tiny planet swings around the frame, hiding half the sky through day and night.

Extra Info (original description):

This timelapse is heavily distorted because it's packing a full spherical image (think globe of stars) and reprojecting it onto a flat surface (your screen). For the first minute or so, the standard "tiny planet" timelapse, the center of the image is straight down - you can see the tripod and the shadow of the camera; the outer edge, literally the entire ring at the edge of the circular image, is straight up, that's why the rim looks so distorted. In fact I made the "planet" a bit smaller as well by throwing in a sqrt(theta) so the edge is even more stretched...

Once the frame switches around and the stars stop moving, I've changed the center of distortion from straight down to straight towards the south pole; the ring at the edge of the frame is now the north pole, and Polaris, the north star, is smeared out over the whole outside of the image. It's kinda weird. However, in this view, the center of distortion is the same as the center of rotation of the stars, so you can spin the frame and none of the stars appear to move!

I like to think of this as the whole sky always being "there", but the earth blocks about half of it ALL THE TIME!

I shot this timelapse over the course of about a week. (the camera was pretty tired and I went through a bunch of 64BG SD cards...) I ended up finally capturing one good 24-hour span with the camera pointed up, and one good 24-hour span with the camera pointed down.

I was using a circular fisheye lens with a 185 degree field of view to capture the entire sky in a single photograph. I also flipped the camera over to take a picture of the ground and capture a full spherical image (albeit separated by a day). I did a whole lot of math and editing in Lightroom (twice for each of the "up" images), Matlab for color temporal smoothing, Davinci Resolve for stabilization and looping, back to Matlab for spherical "unfolding", back to Resolve for compositing the top and bottom images, back to Matlab again for redistorting into the "tiny planet", and finally back into resolve again for a final render. I also passed a few frames through Imagej in order to measure some angles I needed for the polar alignment and "star stabilization". After all that I think it turned out pretty cool!

A bit more detail on the coding:

https://www.reddit.com/r/space/comments/a2dm4v/tiny_planet_in_the_cosmos/eaz5y51/

Camera:

Sony A6000 - Aperture priority, auto-ISO, AWB

Lensbaby circular fisheye (E-mount) - adjusted aperture at sunrise and sunset to control light)

External intervalometer set to 1:20/frame (enough time to process a max 30 second exposure and associated dark frame)

Case-relay power system (plugged into the wall, you can see the orange cable on the ground...)