Summer Solstice 2020

Summer Solstice 2020

In 2020 the Summer Solstice is at 9:44pm UTC on June 20.

We get solstice from the Latin sol (sun) and sistere (to cause to stand) - the moment when the Sun stands still in its journey from north to south and back again.

Summer Solstice is the instant when the Sun is at its highest point in the sky, on the longest day of the year for the Northern hemisphere. This happens because the Earth is tilted by a little over 23 degrees (our planet rotates once a day, but relative to our orbit around the Sun the axis of rotation is at an angle). As we orbit the Sun this tilt means that different latitudes experience more or less sunlight over the course of a year. This pattern is most extreme near the poles. In the Arctic Circle the Sun never sets at the height of summer and never rises in the depth of winter. We mark two solstices each year, summer and winter. At the Summer Solstice the Sun is directly over the Tropic of Cancer (a little over 23 degrees north). We also observe two equinoxes, spring and fall (vernal and autumnal), halfway through the cycle when the Sun is directly over the Equator and a day is the same length everywhere.

The video below shows how the pattern of day and night changes over one year. You can see when the poles are completely dark or light, and the moment when the Sun 'stands still' before days start to get longer or shorter again.

Here's another perspective. This video shows a view from San Francisco made from pictures that each show a complete day (each vertical line on the picture was shot at a different time with noon at the center). You can see the length of day changing throughout the year. On less foggy days you can also see the position of sunset moving, especially with the days getting longer towards the end when San Francisco experiences less fog.

Summer Solstice isn't always on June 20th - sometimes it's June 21st or June 22nd. Irritatingly a day on your clock is not the same as a solar day and a calendar year is not the same thing as one trip around the Sun. This is why we have leap years and leap seconds to stay roughly in sync with celestial mechanics.

It's also interesting to note that Summer Solstice isn't when we're closest to the Sun or when temperatures are the highest. The Earth's orbit is elliptical and we're actually furthest away around the Summer Solstice (for now - this changes over time). Our Northern hemisphere summer is driven by sunlight hitting us directly rather than at an angle (seasons are driven by the 23 degree tilt and the position of the orbit more than our distance from the Sun). Temperatures continue to rise after the Summer Solstice mainly because it takes a while to heat up water, and so warmer weather lags the increase in direct sunlight (and vice versa as we head into colder weather after the Winter Solstice).

The exact moment of Summer Solstice pictured at the start of the post and the video of day/night over a year were created using Catfood Earth. Catfood Earth generates wallpaper from NASA Blue and Black Marble images to show the current extent of day and night combined with near real time cloud cover. Catfood Earth is totally free and available for Windows and Android.


4K One Year Global Cloud Timelapse

Hurricane Dorian in Catfood Earth

Six 4K images a day at 24 frames per second (so each second is four days) from April 18, 2019 to April 17, 2020:

I made a version of this video a couple of years ago using xplanet clouds. That was lower resolution and only had one frame per day so it's pretty quick. This version uses the new 4K cloud image I developed for Catfood Earth just over a year ago. I've been patiently saving the image six times a day (well, patiently waiting as a script does this for me). It's pretty amazing to see storms developing and careening around the planet. The still frame at the top of the post shows Dorian hitting Florida back in September.

Vernal (Spring) Equinox 2020

Spring Equinox 2020 rendered in Catfood Earth

Spring starts right now in the Northern Hemisphere (Autumn if you find yourself south of the Equator). Rendered in Catfood Earth.

Catfood Earth for Android 4.00

Catfood Earth for Android 4.00

Catfood Earth for Android 4.00 is available for download and is updating through the Google Play Store.

As with the 4.00 update for Windows all images have been remastered to 4K resolution. Earth for Android has also been updated to better support Android 10 (updates are faster and the settings layout looks much better). You'll need to grant location permission in settings to have Earth automatically center on your current location. It's also possible to set a center longitude manually (I find -90 works well for centering most of the Americas).

Catfood Earth 4.00

Image from Catfood Earth 4.00

Catfood Earth 4.00 is available for download.

The main change is that all of the images shipped with Catfood Earth have been remastered to 4K resolution. This includes NASA Blue Marble 2 monthly images (which Catfood Earth interpolates daily) and the 2016 version of Black Marble (city lights at night). The Catfood Earth clouds service has been updated to full 4K resolution as well.

Earth 4.00 also includes an update to the 2019c version of the Time Zone Database.

As well as providing desktop wallpaper and a screensaver, Catfood Earth can render frames for any time and date. To celebrate the release of 4.00 I created the 4K video below which shows all of 2019, 45 minutes per frame, 9,855 frames. You'll see the shape of the terminator change over the course of the year (I always post the seasonal changes here: Spring Equinox, Summer SolsticeAutumnal Equinox, and Winter Solstice). If you watch closely you'll also see changes in snow and ice cover and even vegetation over the course of the year.

Winter Solstice 2019

Updated on Sunday, May 3, 2020



It's the start of Winter (or Summer if you're south of the Equator). Rendered in Catfood Earth, showing December 22 at 04:19 UTC.

Autumnal Equinox 2019

Updated on Sunday, May 3, 2020

Autumnal Equinox 2019 in Catfood Earth

Autumn starts now north of the Equator, Spring to the south. Rendered in Catfood Earth.


Summer Solstice 2019

Summer Solstice 2019 in Catfood Earth

Summer starts now in the Northern Hemisphere, Winter if you happen to be south of the Equator. Rendered in Catfood Earth.


Catfood Earth 3.46

Improved clouds in Catfood Earth

Catfood Earth 3.46 is now available for download. Catfood Earth for Android 1.70 is available in the Google Play Store and will update automatically if you already have it installed.

This follows hot on the heels of the last release as the new clouds layer service running on this blog can update far more frequently than the source used prior to 3.45. You will now get a fresh helping of clouds every hour! Unrelated to this release I've improved the quality of the clouds image as well. If you're interested you can read about this in exhaustive detail here.

Improving the accuracy of the new Catfood Earth clouds layer

Equirectangular clouds for Catfood Earth

Last weekend I rushed out a new version of Catfood Earth because the clouds layer stopped working. I'd been using xplanet clouds which published a free 2048x1024 image infrequently and for some reason the site has vanished (probably because it was based on data from the Dundee Satellite Receiving Station which has apparently closed). The University of Wisconsin-Madison Space Science and Engineering Center makes global cloud data available for free for non-commercial use so I plugged this in to get Catfood Earth up and running.

Clouds started working again last Saturday but they were slightly out of position. With a pixel on a HD screen being up to 13 miles this isn't ideal. I hope nobody is using Catfood Earth for weather forecasting or anything too serious but this bothered me enough that I had to fix it. From today the clouds layer is as close to perfect as I can make it.

Mostly for my own sanity in case I ever need to fix this again here's the process.

I use the globalir product because it covers both day and night. This is available in a number of resolutions and you just need to download a set of tiles and stitch them together - I'm using 4096x4096 as the source for the new layer. Catfood Earth uses equirectangular images which have a 2:1 aspect ratio so to start with I resized this less at the equator and more at the poles and got an image that was almost right.

The source image is Spherical (or Web) Mercator which is useful for Google Maps but not right for Catfood Earth. I found a Stack Exchange post which pointed me in the right direction - use GDAL (Geospatial Data Abstraction Library) to transform the Mercator image to equirectangular by tagging the corners and then warping it. This got me closer but it still wasn't right. More googling led me to a post by Robert Simmon with a gdalwarp tip to use -te and -te_srs to force the output to match the dimensions of the input.

The last problem is that the Mercator image doesn't reach all the way to the poles. In this case there is no alternative but to invent some data and so I flip the top and bottom 50 pixels to cover the gap. You can see this if you look carefully but as this region is rendered over ice in Catfood Earth it won't make a difference in practice (and the old image used this trick as well for a greater proportion of the clouds).

One final visual tweak is to drop the lowest intensity pixels which end up blurring out the background in Catfood Earth. You can adjust the transparency and color used to render the layer to suit your tastes (the default is 50% transparency and a gray color for the clouds which I think strikes a good balance).

I also increased the frequency of updates so a new image is available every hour. I said last week that an advantage of this new system was that I wouldn't need to push a new version of Catfood Earth to tweak the cloud logic. That's partly true - you'll get the new improved image with the current version, but I stepped down the update frequency to once a day when I started using xplanet clouds. There will be another update soon to the Windows and Android versions to switch to hourly updates.