Categories: Guide to Space

Solar Day

[/caption]

Since the dawn of time, human beings have relied on the cycles of the sun, the moon, and the constellations through the zodiac in order to measure time. The most basic of these was the motion of the Sun as it traced an apparent path through the sky, beginning in the East and ending in the West. This procession, by definition, is what is known as a Solar Day. Originally, it was thought that this motion was the result of the Sun moving around the Earth, much like the Moon, celestial objects and stars seemed to do. However, beginning with Copernicus’ heliocentric model, it has since been known that this motion is due to the daily rotation of the earth around the Sun’s polar axis.

Up until the 1950’s, two types of Solar time were used by astronomers to measure the days of the year. The first, known as Apparent Solar Time, is measured in accordance with the observable motion of the Sun as it moves through the sky (hence the term apparent). The length of a solar day varies throughout the year, a result of the Earth’s elliptical orbit and axial tilt. In this model, the length of the day varies and the accumulated effect is a seasonal deviation of up to 16 minutes from the mean. The second type, Solar Mean Time, was devised as a way of resolving this conflict. Conceptually, Mean solar time is based on a fictional Sun that is considered to move at a constant rate of 360° in 24 hours along the celestial meridian. One mean day is 24 hours in length, each hour consisting of 60 minutes, and each minute consisting of 60 seconds. Though the amount of daylight varies significantly throughout the year, the length of a mean solar day is kept constant, unlike that of an apparent solar day.

The measure of time in both of these models depends on the rotation of the Earth. In both models, the time of day is not plotted based on the position of the Sun in the sky, but on the hour angle that it produces – i.e. the angle through which the earth would have to turn to bring the meridian of the point directly under the sun. Nowadays both kinds of solar time stand in contrast to newer kinds of time measurement, introduced from the 1950s and onwards which were designed to be independent of earth rotation.

We have written many articles about Solar Day for Universe Today. Here’s an article about how long a day is on Earth, and here’s an article about the rotation of the Earth.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Solar_time
http://www.tpub.com/content/administration/14220/css/14220_149.htm
http://scienceworld.wolfram.com/astronomy/SolarDay.html
http://www.britannica.com/EBchecked/topic/553052/solar-time?anchor=ref144523
http://en.wikipedia.org/wiki/Hour_angle

Matt Williams

Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.

Recent Posts

SpaceX Reveals the Beefed-Up Dragon That Will De-Orbit the ISS

The International Space Station (ISS) has been continuously orbiting Earth for more than 25 years…

19 hours ago

Gaia Hit by a Micrometeoroid AND Caught in a Solar Storm

For over ten years, the ESA's Gaia Observatory has monitored the proper motion, luminosity, temperature,…

2 days ago

Lunar Infrastructure Could Be Protected By Autonomously Building A Rock Wall

Lunar exploration equipment at any future lunar base is in danger from debris blasted toward…

2 days ago

Why is Jupiter’s Great Red Spot Shrinking? It’s Starving.

The largest storm in the Solar System is shrinking and planetary scientists think they have…

3 days ago

ESA is Building a Mission to Visit Asteroid Apophis, Joining it for its 2029 Earth Flyby

According to the ESA's Near-Earth Objects Coordination Center (NEOCC), 35,264 known asteroids regularly cross the…

3 days ago

The Most Dangerous Part of a Space Mission is Fire

Astronauts face multiple risks during space flight, such as microgravity and radiation exposure. Microgravity can…

3 days ago