What Is Solar Noon?
Solar noon is the moment when the Sun passes a location's meridian and reaches its highest position in the sky. In most locations, it doesn't happen at 12 o'clock.
What Is Noon?
The English language is a little imprecise when it comes to the word “noon”. It can mean 2 different things:
- In terms of civil time, noon is simply another word for 12 o'clock, the moment separating morning and afternoon. As such, it is the opposite of midnight. Alternative names include midday and noon time.
- In terms of solar time, noon is the moment when the Sun crosses the local meridian and reaches its highest position in the sky, except at the poles. This version of noon is also called solar noon or high noon.
Meridians and the Sun
A meridian is an imaginary line running from the North Pole to the South Pole along the Earth's surface. It connects all locations that share the same longitude, meaning that they are exactly north or south of each other. The line running from one pole to the other via your location is your local meridian.
Solar noon happens at your location when the Earth's rotation brings your local meridian to the side of the planet that faces the Sun. From your perspective, the Sun, after having steadily gained altitude since sunrise, now reaches the top of the arch that its journey describes in the sky every day. At this moment, it appears due south, due north, or in the zenith position exactly above you, depending on your latitude and the time of year.
Since solar time depends on the longitude, solar noon occurs at exactly the same moment in all locations that share your local meridian.
When Is Solar Noon?
In most places on Earth, solar noon does not happen at 12 o'clock. The Earth's rotation slowly shifts the meridian experiencing solar noon from east to west. In other words, solar noon happens a little earlier in locations just east of you and a little later in locations west of you.
Since our clocks are set according to time zones, civil time changes abruptly as you move from one time zone to another, usually in 1-hour increments. While this undeniably makes life easier for us, it does not reflect the even movement of the Earth's rotation and the gradual geographical progression of local solar time.
This means that clocks in the eastern part of each time zone show an earlier time at solar noon than clocks near its western border. Even if time zones were used the way they were once envisioned—where local time is based on the solar time in the zone's center, with the time zone extending 7.5 degrees of longitude to the west and to the east of the center line—solar noon would occur at 11:30 (11:30 am) at the eastern time zone border and at 12:30 (12:30 pm) at the western border.
Latest Solar Noon in China
In real life, this difference is even larger because time zones rarely follow this ideal. Their borders are often grossly distorted by political or geographical factors. For example, China spans more than 60 degrees of longitude but the country follows a single time zone. This means that solar noon in western areas occurs later than 15:00 (3:00 pm) during some parts of the year, later than anywhere else on Earth.
In many countries, Daylight Saving Time (DST) further increases the discrepancy between civil and solar time.
Elastic Solar Time
Of course, if you happen to live in a location whose solar time is used as the basis for civil time in your time zone, solar noon will happen at or around 12 o'clock for you. But even that is only true during some parts of the year. The Earth's rotation and its movements in relation to the Sun are not quite constant, so the length of a solar day, which is the time span from one solar noon to the next, varies during the course of a year. This phenomenon is referred to as the equation of time.
For example, around the December solstice, solar noons are a bit farther apart, so the Sun crosses the local meridian a little later every day. During other parts of the year, for example in the time following the June solstice, solar noon happens a little earlier each day and solar days are a little shorter.
Solar Noon at the Poles
All meridians converge at the North Pole and the South Pole. So, unlike any other location on Earth, the poles don't have a longitude. By extension, there is no solar noon because there is no meridian the Sun can cross.
In practice, the Sun does not go up and down on a daily basis like everywhere else on Earth. Rather, its altitude in the sky constantly increases during the winter and spring and decreases during the summer and autumn (fall), creating 6 months of polar night, followed by 6 months of midnight Sun. At the North Pole, for example, the Sun reaches its lowest altitude at the December solstice, which marks the beginning of winter there, and its highest position at the June solstice, when summer starts.
Where Does the Word Noon Come From?
Its origin lies in the Latin word none, referring to the 9th hour after daybreak. Originally, it was used to denote the timing of a daily prayer or meal at 15:00 (3:00 pm), nine hours after 06:00 (6:00 am). In the 12th century, the prayer and meal were shifted to 12:00 (12:00 pm), while the term none remained the same, inspiring the use and timing of today's noon.
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- International Atomic Time (TAI)
- Universal Time (UT)
- Local Mean Time (LMT)
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