Story Index:

This motorcyclist has guts!
05/21/2023

Saving History
01/16/2023

Cool little Photosphere trick!
10/10/2018

Eclipse!!
08/29/2017

Summer Weekend in the Colorado Rockies
07/19/2016

Is there a Spring Cleaning for the mind?
05/16/2016

Kim's Day In The Sun
02/28/2015

Tracing Roots
01/15/2015

Whitney Completes Her Master's Degree
01/10/2015

Day Trip to Moorea
11/01/2012

Stopover in Tahiti
10/30/2012

Exploring New Zealand's Northland
10/27/2012

Waiheke Island
10/24/2012

2012 Triathlon World Championships
10/23/2012

Travails with Tom-Tom
10/21/2012

Off to New Zealand!
10/18/2012

Cycling on the Oregon Coast
07/24/2012

Partial Solar Eclipse, May 20
05/23/2012

"Tour of California" Bike Race comes through town!
05/15/2012

Wildflower Triathlon a family affair
05/07/2012

Truth in Packaging?
04/01/2012

Nik Graduates from Cal Poly
12/10/2011

Cal Poly's Open House Weekend
04/17/2011

The 2010 US Census is here
03/16/2010

Cultural Musings...
09/18/2009

2009 Triathlon World Championships
09/12/2009

Sightseeing around the Gold Coast
09/09/2009

South Bank: Brisbane's Fine Arts center
09/05/2009

Brisbane, Australia
09/04/2009

Catching up . . .
08/31/2009

Summa Cum Laude, Baby!
06/15/2008

Kim completes her first Half-Ironman
06/10/2008

Kim Visits Home for Wildflower Triathlon
05/16/2008

The Last Supper
02/16/2008

Farewell, Dad
01/15/2008

Whitney Plays Final Soccer Game
12/10/2007

Watch the Women's World Cup!
09/12/2007

Kim Blazes Chicago Triathlon
08/27/2007

Nik Bicycles Pacific Coast
08/15/2007

Whitney Begins Final Season
08/13/2007

More Triathlons for Kim
06/15/2006

Corvallis gets snow!
03/09/2006

Pac-10 Conference Player of the Week
01/15/2006

Whitney's new cat
09/26/2005

Kim in Chicago Triathlon
08/31/2005

Rare family get-together
08/23/2005

Empty Nest
08/11/2005

Whitney Signs National Letter-of-Intent
02/08/2004

ODP cancels European tour
04/11/2003

Whitney renamed to Regional Team
08/07/2002

Kim graduates from Willamette
06/17/2002

Whitney Stars in High School Soccer
03/07/2002

Nik graduates from UTI
01/07/2002

2001 - A Year for Remembrance
12/30/2001

Whitney selected to the 2001 State ODP pool for U-15 Girls
03/22/2001

Kim returns from Europe
02/02/2001

2000: A Year for Traveling
12/15/2000

Kim Travels Europe
11/24/2000

Whitney selected to the US Far Western Regional ODP Pool!
07/14/2000

Congratulations to Nik, Ygnacio Valley High Graduating Class of 2000!
06/30/2000
Cool little Photosphere trick!

There's a photo making the rounds on Reddit and Twitter that's generating a bit of confusion. Some people are believing this photo is a time-lapse of the moon, photographed at the same time on 27 consecutive days, and depicting how the moon moves in a figure-8 pattern. *


Composite image of lunar phases by Giorgia Hofer
I'm not on Reddit or Twitter, and my post isn't about that photo, but that's just a preface to explain how I got involved in the photosphere project described below. Knowing my long involvement with astronomy and mathematics, my son alerted me to the controversial photo and asked for my opinion. **

As part of my response to him, I decided to see if I could illustrate, over a period of consecutive days, the moon's path in VR format. So that's what this post is about.

Calling upon my knowledge of spherical trigonometry and celestial calculations, I drew up a spreadsheet to convert the usual astronomical coordinates (Right Ascension and Declination) into the more pedestrian and earth-centered Azimuth and Elevation coordinates. ***

This would allow me to overlay the moon's position onto a Google photosphere image for viewing in a VR viewer.

You can see the result here. If you have Google Chrome or a compatible browser, click the image and it should appear in 3D/VR mode, in which you can use your mouse to pan around for a 360-degree view. If you have a VR viewer like Google Cardboard or similar, you can load the image on your phone and view it that way for a cool, immersive experience.

The photosphere I chose is one I created in August 2017, just before the total solar eclipse. The location was Corvallis, Oregon, almost at latitude 45 degrees, and the time of day was about 10:30 in the morning. On this image I overlaid the position of the moon, relative to the sun's position, as of 10:30am on the 25 days preceding the eclipse.

(My photosphere wasn't of the eclipse, but the eclipse isn't relevant anyway; that's not the point. The point was to illustrate the relative locations of the sun and moon at the same time of day, over several consecutive days. The fact that the photosphere I chose to work with happened to be near the date and location of the eclipse is basically coincidental.)

Editing the photosphere turned out to be an intriguing and fun exercise!

Using my phone's camera to capture a photosphere resulted in an 8000x4000 jpeg image. Metadata within the jpeg instructs compatible viewers to display the image in a sphere. 8000 pixels corresponds to 360 degrees in the horizontal direction, and 4000 pixels corresponds to 180 degrees vertically (90 degrees from zenith to horizon plus 90 degrees from horizon to nadir).

Using a little math and an ordinary graphics editor, I turned the 25 azimuth/elevation coordinates into pixel x/y coordinates and overlaid small jpeg images of the moon. (I found an image online of moon phases, and cut-and-pasted those.)

It's pretty straightforward in the horizontal direction, but the 180 vertical degrees are mapped into the photosphere image as an orthographic projection. Readers will be familiar with orthographic projection as what makes arctic lands like Greenland and Finland appear larger on world maps than they actually are.

As a result, I'd need to add some intentional distortion in the cut-and-pasting to make things appear correctly in VR. For instance, in an orthographic projection an object will span twice as many pixels (horizontally) at an elevation of 60 degrees as it would at horizon level. That means I would need to distort my little moon graphic in the horizontal direction before pasting it over the photosphere image. A little plane trigonometry [1/cos(elev)] would tell me how much distortion I would need to build in.

Once all my little moon graphics were pasted in and the photosphere jpeg resaved, I discovered it no longer worked as a VR photosphere because my graphics editor had stripped out the VR metadata. A little online searching, however, led me to "Exif Fixer", a neat little one-trick utility that adds the metadata back into the jpeg file.

The edited photosphere turned out better than I expected, and the result looks kind of cool when viewed in a VR viewer!

I thought I'd post this in case others are as intrigued as I am with the possibilities.


Footnotes:

* The photographer never stated the photo was a time-lapse, and that should be obvious from the photographer's detailed description. But unsurprisingly, most of the reposts to Twitter and Reddit omitted the photographer's description, thus leaving readers to jump to conclusions.

** Long story short, the photo is just a Photoshopped, artistic creation. That kind of time-lapse photo would be impossible in the physical world. The moon's path doesn't twist like that.

*** I hate "reinventing the wheel" if I don't have to, so I tried a number of online astronomical calculators. That proved fruitless, however, because none would calculate the azimuth and negative elevation when the moon was below the horizon, and that's what I need for this project. Hence, the need to write my own spreadsheet to do the calculations.


Posted by Dan 10/10/2018
FreeStyle Journal 19.03.21
©2003-2011 by Dan Goodell

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