Although there’s currently snow on the ground and my outdoor thermometer was hovering around 20 degrees this morning, I keep saying to myself, “there’s no turning back now.” And indeed, spring is coming. The solar terminator will be perpendicular to the equator, the sun will cross the celestial equator, and the sun’s ecliptic longitude will reach zero. And this will all happen on March 20th, regardless of any amount of snow out my window. The vernal equinox, which marks the astronomical beginning of spring in the Northern Hemisphere, will occur on Monday at 10:28 Universal Time (that’s 5:28am for those of us in Central Daylight Time). And here’s what that means:

The Solar Terminator: The solar terminator is the edge between night and day. On the equinox, this edge is perpendicular to the equator, and the Northern and Southern Hemispheres are equally illuminated (as in the main photo above). This also means that for most locations on Earth, day and night will be equal lengths. (Although, there’s one quick side note: because the sun is a disk, not a point, the sunrise is marked by the leading edge of the sun appearing, and the sunset is marked by the trailing edge disappearing. This gives us about three more minutes of day than night.) And yes, the sun will rise and set in due east and west – read more here.

The Celestial Equator: Imagine the sky as a dome which rotates around us. Now, project the Earth’s equatorial line out onto that dome — this is the celestial equator. We can further add longitudinal lines (known as right ascension) and latitudinal lines (known as declination) to our dome, and label them (right ascension is labeled in hours (24) and declination is labeled in degrees above or below the equator). On the vernal equinox, from a viewpoint on Earth, the sun crosses the celestial equator from the Southern Hemisphere into the Northern Hemisphere.

The Sun’s Ecliptic Longitude: According to HyperPhysics, “the apparent path of the Sun’s motion on the celestial sphere as seen from Earth is called the ecliptic. The ecliptic plane is tilted 23.5° with respect to the plane of the celestial equator since the Earth’s spin axis is tilted 23.5° with respect to its orbit around the sun.” The two planes intersect at the vernal and autumnal equinox. If we label the ecliptic with longitudinal lines, the vernal point is the origin — or zero degrees. So, this Monday, at 5:28am, the sun will reach zero degrees along the ecliptic.

To see the solar terminator in action, check out this view of shifting sunlight throughout the year – in 12 seconds.


To Do This Month:

Visit Milwaukee’s Mitchell Park Domes for a Spring Equinox and Green Living Event on Sunday, March 19th.

Burn your socks as per this Maryland tradition that celebrates the fashion rule that socks shall not be worn with deck shoes after the vernal equinox.

Take advantage of the springtime’s freezing nights and warmer days by collecting the sap of a sugar maple tree as it flows through the xylem. Find out what this looks like and how climate change may be affecting the syruping season with this video from Climate Wisconsin:

Check out the many events happening at Nature Net sites this month on our joint Events Calendar, including the UW Arboretum’s Spring Equinox Night Walk on Monday, March 20th, and the Aldo Leopold Nature Center’s Maple Syrup Fest on Sunday, March 26th.

March Events

Our Favorite Spring Equinox Books

Jackson School Forest

Recycled Can Bird Feeder

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For Educators:

The Physics Classroom

Considering the fact that “every satellite’s motion is governed by the same physics principles and can be described by the same mathematical equations” it certainly would be useful – in the endeavor to teach about the equinox and the Earth’s orbit around the sun – to have a reliable source for said principles and equations. A resource that also provides teacher tools and online interactives for students would be even better. Lo, such a resources does exist: the Physics Classroom. This free, online resource provides tutorials for teachers (including a subsection that allows you to test your own knowledge), “interactives” designed for students to use on tablets or Chromebooks, and animations and QuickTime movies that demonstrate physics principles in a visual manner.
The Physics Classroom has also created an app called Minds On Physics (MOP) where students get immediate feedback and assistance, and teachers get progress reports and insight into how students are advancing through the lessons.
There’s plenty of classroom worksheets and assistance in aligning Next Generation Science Standards with curricula.

If this high-school level resource is not for you, the American Physical Society has a few resources for K-8 physics, as does

For Families:

Spring Phenology

Aldo Leopold once wrote, “one swallow does not make a summer, but one skein of geese, cleaving the murk of March thaw, is the spring.” Leopold is famous for keeping phenological records of natural events throughout the seasons. Many of which are triggered by the amount of daylight, including bird migration and the flowering of many plants. So, although there is snow and sleet today, day length is inevitably changing and impacting the ecosystem. Here are a few things Leopold and his family were watching for this time of year – you can watch (or listen) for them too:

  • Black-capped chickadee – singing two-note song
  • Wisconsin River opens up
  • Striped skunk – first tracks
  • Northern cardinal – first song
  • Canada goose – spring arrival
  • American robin – first arrival
  • Eastern bluebird – first arrival
  • Turkey vulture – spring arrival
  • Sandhill cranes – spring arrival
  • Red-winged black bird – spring arrival
  • And check out Nature Net’s May 2016 issue for spring wildflowers to watch for

If you do see these signs of spring, you can enter them into Nature’s Notebook, a citizen science project run by the National Phenological Network.


Betsy bylineCopy of Betsy bylineBetsy Parker is an environmental educator who supports all children, families, and classrooms getting their recommended daily allowance of #VitaminN.
Funding for Nature Net and the Nature Net News blog is provided by American Girl Fund for Children.