Explanation: When does the line between day and night become vertical? Tomorrow. Tomorrow is an equinox on planet Earth, a time of year when day and night are most nearly equal. At an equinox, the Earth's terminator -- the dividing line between day and night -- becomes vertical and connects the north and south poles. The featured time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the Sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere -- and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken -- and will take -- around the Sun.
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 8:15 am
by bjmb
very good timr-lapse video. yet: "a time of year when day and night are most nearly equal. " - almost there: both poles and adjacent areas bathe in 24 hr sunlight, no night there.
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 11:44 am
by RedFishBlueFish
Wonderful visualization and demonstration of the origin of the seasons.
Wonderful if this clip would be shown in schools on the Quarter Days of the Wheel of the Year.
Somehow I missed seeing it in 2014 when it was posted: Showing value of APOD Sunday greats!
Thanks
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 1:09 pm
by E Fish
That's amazing. While I know the definition of an equinox, I don't think I've ever seen a video showing the actual movement of the terminator. Definitely going to show this to my students. We're just about to start the astronomy unit anyway.
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 1:17 pm
by Guest
Very interesting. In addition to the terminator excursions, I found the cloud patterns fascinating. It appears the video is centered on Africa and one can sea the dearth of clouds throughout the year over the Sahara Desert.
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 2:49 pm
by Steve Dutch
And here I thought "equinox" mean all the cylinders in my car were misfiring equally badly!
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 2:53 pm
by Chris Peterson
Steve Dutch wrote:And here I thought "equinox" mean all the cylinders in my car were misfiring equally badly!
I checked Google the other day to find the exact time of the equinox, and the top three items that came up were for some car I've never heard of! That's just sad.
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Sun Mar 19, 2017 5:47 pm
by De58te
I looked up the 'equinox definition' on Yahoo search and I got "a hybrid species resulting from the interbreeding of the horse family and the oxen family."
{Early April Fool]
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Mon Mar 20, 2017 11:33 am
by Guest
I've understood intellectually about the tilt of the Earth's axis and the seasons "for ever", but to actually see it happening before my eyes is just amazing. Thanks!
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Fri Mar 24, 2017 5:09 pm
by Not Blinky
Clear skies over the Sahara most of the year. Are there any observatories there?
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Fri Mar 24, 2017 5:17 pm
by Chris Peterson
Not Blinky wrote:Clear skies over the Sahara most of the year. Are there any observatories there?
Keep in mind that clear skies are not the only factor considered when planning an observatory. There are many places with clear skies. Also very important (often more important) is a shallow water column (so high altitude) and very good seeing (determined by local and regional geography).
Re: APOD: Equinox on a Spinning Earth (2017 Mar 19)
Posted: Fri Mar 24, 2017 5:39 pm
by neufer
Chris Peterson wrote:
Not Blinky wrote:
Clear skies over the Sahara most of the year. Are there any observatories there?
Keep in mind that clear skies are not the only factor considered when planning an observatory. There are many places with clear skies. Also very important (often more important) is a shallow water column (so high altitude) and very good seeing (determined by local and regional geography).
ESO 2005 Astronomy & Astrophysics
Optical seeing monitoring at the Oukaïmeden
in the Moroccan high atlas mountains.
<<The renewed interest in ground-based astronomy and the developing worldwide search for sites with good seeing for the next generation of the Extremely Large Telescopes (ELT), led us to develop an instrument to monitor astronomical seeing. The possible selection of the Moroccan High Atlas mountains as a potential site for ELT’s encouraged us to install this instrument at Oukaïmeden for testing its capability and to complete the qualification studies of this site as a first candidate in the High Atlas site search. Our plan is to build several copies of this instrument and set them in different locations to assist us in the next site prospection campaigns.
Oukaïmeden site is located at 2700 m above sea level, 70 km from Marrakesh city. Statistics of 15 months of seeing at Oukaïmeden, between the July 2003 and September 2004, have been presented. Median and mean seeing values of 0.84 arcsec and 0.92 arcsec, respectively, were achieved over the campaign. The results presented in this study have made it possible to enhance the qualification campaign of the Oukaimeden site started several years ago (Benkhaldoun 2000). The median seeing we found led us to conclude that the Oukaimeden is among the best observatory sites. This study also shows that there is no obvious seasonal effect. Prospection of other sites in the Morrocan Atlas Mountains should also continue, since some may have even better seeing due to their location relative to the dominant wind flow.>>
Equinox on a Spinning Earth