# Having fun with the equation of time

##### July 28, 2014 by David Dickinson

If you're like us, you might've looked at a globe of the Earth in elementary school long before the days of Google Earth and wondered just what that strange looking figure eight thing on its side was.

Chances are, your teacher had no idea either, and you got an answer such as "it's a calendar, kid" based on the months of the year marking its border.

In a vague sense, this answer is correct… sort of. That funky figure eight is what's known as an analemma, and it traces out the course of the Sun in the sky through the year as measured from a daily point fixed in apparent solar time.

But try explaining that one to your 3rd grade teacher. Turns out, measuring the passage of time isn't as straight forward as you'd think. Our modern day clock and calendar is a sort of compromise, a method of marking the passage of time in a continuing battle to stay in sync with the heavens.

For most of history, the daily passage of time was denoted by the Sun. Solar Noon occurs when the Sun stands at its highest elevation (also known as its altitude) above the local horizon when it transits the north-south meridian. The trouble is, the passage apparent solar time doesn't exactly match what we call solar mean time, or the 24 hour rotation of the Earth. In fact, this discrepancy can add up to as much as more than 16 minutes ahead of solar noon in late October and November and over 12 minutes behind it in February. This is worth bringing up this week because this factor, known as "The Equation of Time"—think "equation" in the sense that sundial owners must factor it in to make solar mean and apparent time "equal"—reaches its shallow minimum for 2014 this Saturday at 7:00 UT/3:00 AM EDT with a value of -6.54 minutes.

So, what gives? Why won't the pesky universe stay in sync?

Well, the discrepancy arises from two factors: the eccentricity of the Earth's orbit, or how much it deviates from circular and the obliquity of the ecliptic to the celestial equator, think the tilt of Earth's axis. Of the two, obliquity is the major factor, with eccentricity playing a minor but measurable role. And remember, we move slightly faster in our orbit in January near perihelion as per Kepler's Laws of planetary motion than at aphelion, which occurred earlier this month , though be careful not to confuse the term "faster" with " fast."

This means that were the Earth to orbit the Sun in a perfect circle with its poles perpendicular to its orbit, apparent and mean time would essentially stay in sync. Of course, no known planet has such a perfect alignment scenario, and other worlds do indeed host alien analemmas (analemmae?) of their own.

It's also interesting to note that the two each major and minor minima of the Equation of Time roughly coincide with the four cross quarter tie in days of the year (marked by Groundhog's Day, May Day, Lammas Day and Halloween, respectively) while the zero value points fall within a few weeks of the equinoxes and solstices.

In the current epoch, the deep minimum falls on February 21st, while the highest maximum falls on November 3rd on non-leap years. The four zero value dates are April 15th, June 13th, September 1st and December 25th respectively. The exact timing of these also slip to the tune of about a second a year, but of course, most sundials lack this sort of precision.

So, why should we care about the Equation of Time in the modern atomic clock age? It is true that there have been calls over the past few years to "abolish the leap second" and go off of the astronomical time standard entirely… if this ever does come to pass, some future Pope Gregory will have to institute a "leap hour" circa 10,000 A.D. or so to stop the Sun from rising at 2 AM. But some modern day Sun tracking devices (think heliostats or solar panels) do in fact use mechanical timers and must take the equation of time into account to maximize effectiveness.

Want to see the Equation of Time in action? You can make your own analemma simply by photographing the position of the Sun at the same time each day. Just remember to account for the shift on and off of Daylight Saving if you live in an area that observes the archaic practice, residents of Arizona need not to take heed. Otherwise, you'll end up with a "split analemma…" Wintertime near the December Solstice is the best time to start this project, as the Sun is at its lowest noonday culmination and this will assure that your very own personal analemma won't fall below the local horizon.

Farther afield, the effects of the Precession of the Equinoxes will also tweak the dates of the Equation of Time values a bit. Live out a full 72 year life span, and the equinoctial points will have drifted along the ecliptic by about one degree, twice the diameter of the Full Moon. Incidentally, the failure to take Precession into account is yet another spectacular fail of modern astrology: most "houses" or "signs" have drifted in the past millennia to the point where most "Leos" are in fact "Cancers!"

Such is the challenges and vagaries of modern day astronomical time-keeping. Let us know of your tales of tragedy and triumph as you hunt down the elusive analemma.

Explore further: Leap years prevent 'calendar climate change'

## Related Stories

#### Leap years prevent 'calendar climate change'

March 1, 2012

Without leap years, Earth would experience "calendar climate change" and the seasons would completely swap every 750 years, a Queensland University of Technology scientist says.

#### SDO observes Earth, lunar transits in same day

March 11, 2013

On March 2, 2013, NASA's Solar Dynamics Observatory (SDO) entered its semiannual eclipse season, a period of three weeks when Earth blocks its view of the sun for a period of time each day. On March 11, however, SDO was treated ...

#### Another word for Mars is dust, says rover camera operator

May 6, 2014

(Phys.org) —There's a four-letter word to describe conditions on Mars, and it's not pretty: Dust. It is everywhere and anywhere on Mars, and dust is a key component of Martian weather, says a Texas A&M University researcher ...

#### Sun-like stars reveal their ages

July 10, 2014

(Phys.org) —Defining what makes a star "Sun-like" is as difficult as defining what makes a planet "Earth-like." A solar twin should have a temperature, mass, and spectral type similar to our Sun. We also would expect it ...

#### Suddenly, the sun is eerily quiet: Where did the sunspots go?

July 21, 2014

The sun has gone quiet. Almost too quiet. A few weeks ago it was teeming with sunspots, as you would expect since we are supposed to be in the middle of solar maximum-the time in the sun's 11-year cycle when it is the most ...

#### Earth survived near-miss from 2012 solar storm: NASA

July 25, 2014

Back in 2012, the Sun erupted with a powerful solar storm that just missed the Earth but was big enough to "knock modern civilization back to the 18th century," NASA said.

## Recommended for you

#### Getting into the flow on the International Space Station

December 1, 2015

Think about underground water and gas as they filter through porous materials like soil and rock beds. On Earth, gravity forces water and gas to separate as they flow through the ground, cleaning the water and storing it ...

#### LISA Pathfinder will pave the way for us to 'see' black holes for the first time

December 1, 2015

It has been a century since Einstein presented his theory of general relativity but it is still helping us unveil some of the deepest mysteries of the universe. Now the LISA pathfinder mission will prepare the way for us ...

#### Exiled exoplanet likely kicked out of star's neighborhood

December 1, 2015

A planet discovered last year sitting at an unusually large distance from its star - 16 times farther than Pluto is from the sun - may have been kicked out of its birthplace close to the star in a process similar to what ...

#### Cassini mission provides insight into Saturn

December 1, 2015

Scientists have found the first direct evidence for explosive releases of energy in Saturn's magnetic bubble using data from the Cassini spacecraft, a joint mission between NASA, the European Space Agency, and the Italian ...

#### Timing a sextuple quasar

December 1, 2015

Quasars are galaxies with massive black holes at their cores around which vast amounts of energy are being radiated. Indeed, so much light is emitted that the nucleus of a quasar is much brighter than the rest of the entire ...

#### Scientists get first glimpse of black hole eating star, ejecting high-speed flare

November 26, 2015

An international team of astrophysicists led by a Johns Hopkins University scientist has for the first time witnessed a star being swallowed by a black hole and ejecting a flare of matter moving at nearly the speed of light.

## 2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

##### Doug_Huffman
1 / 5 (3) Jul 28, 2014
The analemma image from Hanks' Cast Away should have been used - and explained.
##### loz
not rated yet Jul 28, 2014
The correct plural of analemma is analemmata, < (Greek) analemma, neuter, genitive stem analemmatos; plural = analemmata; < ana + lepo (verb) "to peel up".

feliciter.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.