Autumn 2024: When does it start, and what is the equinox?
Autumn starts this year on Sunday 22 September and ends on 21 December 2024. Or does it? You may have heard us already talking about autumn in the weather forecasts.
This is because the astronomical autumn begins at the autumn equinox (22 September this year) but the meteorological one always starts on 1 September. For meteorological records it is preferable to group the seasons into three-month periods with the northern hemisphere autumn being September, October and November. This makes year-on-year comparisons easier. The word "equinox" is derived from Latin and literally translates to "equal night". On these days, everywhere on Earth experiences roughly 12 hours of sunshine and 12 hours of darkness.
The Earth's axis is titled at an average of 23.5°. As the Earth travels on its year-long path around the Sun it is tilted towards or away from the Sun, which gives us our seasons. But, on the day of an equinox, the tilt of Earth's axis is perpendicular to the Sun's rays, producing a nearly equal amount of day and night all over the world. Normally, the autumn equinox falls between 22 and 24 September, and this year the equinox occurs on 22 September at 13:44 BST.
Illumination of Earth by the Sun on the day of an equinox
The September equinox (or Southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox can occur at any time from September 21 to 24.
At the equinox, the Sun as viewed from the Equator rises due east and sets due west. Before the Southward equinox, the Sun rises and sets more northerly, and afterwards, it rises and sets more southerly.
The equinox may be taken to mark the end of summer and the beginning of autumn (autumnal equinox) in the Northern Hemisphere, while marking the end of winter and the start of spring (vernal equinox) in the Southern Hemisphere. The September equinox is one point in time commonly used to determine the length of the tropical year.
read more
What is an Equinox?
read more
An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about of equal length. The two equinoxes occur around March 20–21 and September 22–23.
The equinoxes occur on these days, but an equinox is not the whole day—it is the moment when the sun is directly above Earth’s equator. The word equinox can also refer to the position of the sun at this moment. This can also be called the equinoctial point. The equinoxes are traditionally considered to mark the start of spring and fall. In the Northern Hemisphere, the vernal equinox (or spring equinox) occurs in March and the autumnal equinox occurs in September. In the Southern Hemisphere, it’s the reverse.
In contrast, a solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day. There are two solstices during the year: one that occurs around June 20–22 (usually June 20 or 21) and one that occurs around December 20–23 (usually December 21 or 22). In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December. In the Southern Hemisphere, it’s the reverse.
read more
Lahaina Noon - Zero Shadow Day 19 Sep 2024
Lāhainā Noon, also known as a zero shadow day, is a semi-annual tropical solar phenomenon when the Sun culminates at the zenith at solar noon, passing directly overhead (above the subsolar point). As a result, the sun's rays will fall exactly vertical relative to an object on the ground and cast no observable shadow.
A zero shadow day occurs twice a year for locations in the tropics (between the Tropic of Cancer at latitude 23.4° N and the Tropic of Capricorn at 23.4° S) when the Sun's declination becomes equal to the latitude of the location, so that the date varies by location. The term "Lāhainā Noon" was coined by the Bishop Museum in Hawaiʻi. The subsolar point travels through the tropics. Hawaiʻi is the only US state in the tropics and thus the only one to experience Lāhainā Noon. In 2022 and 2023, the phenomenon occurred in Honolulu on May 26 and July 16. Hawaii and other locations between the Tropic of Cancer and Tropic of Capricorn receive the sun's direct rays as the apparent path of the sun passes overhead before and after the summer solstice.
Lāhainā Noon can occur anywhere from 12:16 to 12:43 p.m. Hawaii–Aleutian Standard Time. At that moment objects that stand straight up (flagpoles, bollards, telephone poles, etc.) cast no outward shadow. The most southerly points in Hawaii experience Lāhainā Noon on earlier and later dates than the northern parts. For example, in 2001 Hilo on the Island of Hawaiʻi encountered the overhead sun around May 18 and July 24, Kahului, Maui, on May 24 and July 18, Honolulu, Oʻahu, on May 26 and July 15 and Līhuʻe, Kauaʻi, on May 31 and July 11. Between each pair of dates, the sun is slightly to the north at solar noon. Chosen in a contest sponsored by the Bishop Museum in the 1990s, Lāhainā Noon was the selected appellation because lā hainā (the old name for Lāhainā, Hawaii) means "cruel sun" in the Hawaiian language. The ancient Hawaiian name for the event was kau ka lā i ka lolo which translates as "the sun rests on the brains.
