Equinox explained<\/strong><\/h2>An equinox is one of two moments in a planet\u2019s orbit around the Sun when its axis of rotation \u2013 in Earth\u2019s case, the line connecting the north and south poles \u2013 points neither towards nor away from the Sun.<\/p>
Astronomically speaking, autumn begins at the September (or southward) equinox.<\/p>
In 2024, the centre of the Sun crosses the celestial equator on Sunday 22 September at 12:44pm (13:44 UTC), marking the Northern Hemisphere\u2019s autumn equinox.<\/p>
At this point the Sun moves from the northern celestial hemisphere to the southern celestial hemisphere.<\/p>
The September equinox also marks which full Moon<\/a> is determined as the Harvest Moon<\/a> for that year.<\/p>Those who observe the southern hemisphere sky<\/a> mark the advent of spring on this date, which is their vernal rather than autumnal equinox.<\/p>![\"Sunrise](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/09\/GettyImages-136251263-61669a1-e1599722936126.jpg\" "\\"\\"\/")
Sunrise on Autumn Equinox from Glastonbury Tor, Somerset, UK. Credit: James Osmond \/ Getty Images<\/figcaption><\/figure>What the autumn equinox 2024 means<\/strong><\/h2>The term equinox is derived from the Latin for \u2018equal night\u2019<\/p>
It reflects how on these dates, outside of the polar regions, the Sun will be above the horizon for as long as it is below it.<\/p>
So why the extra daylight, which ranges from a few minutes at the equator to over 20 minutes nearer the poles?<\/p>
Part of the answer is that we see the Sun as a resolved disc measuring 0.5\u00ba across rather than as a point source.<\/p>
Astronomical convention defines the sunrise and sunset times as the first and last moments that the solar disc can be seen, rather than when its centre crosses the horizon.<\/p>
Sunrise and sunset are therefore earlier and later, respectively, than the times at which the Sun is aligned with the horizon.<\/p>
This \u2018extra\u2019 daytime is gained twice per day as the 0.25\u00ba between the limb and the centre of the Sun rotates through the horizon.<\/p>![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/09\/Sunset-a5fd6a3.jpg\" "\\"\\"\/")
The fact that we consider sunset to be when the last vestige of the solar disc drops below the horizon adds minutes to the day. Credit: iStock<\/figcaption><\/figure>Refraction and daylight extension<\/strong><\/h2>Daytime is extended even further by the refraction of sunlight in Earth\u2019s atmosphere.<\/p>
Refraction<\/a> is the bending of waves as they travel through materials of different density, as sunlight does when entering our thick atmosphere from interplanetary space.<\/p>When we view the Sun near the horizon, we see it through a layer of atmosphere that bends its light toward us by a full 0.5\u00ba, making it appear higher in the sky than it actually is.<\/p>
This means that at dawn, we see the full disc of the Sun above the horizon when it is still entirely below it!<\/p>
More \u2018extra\u2019 daytime is needed for Earth to rotate through this additional angle.<\/p>![\"There](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/04\/Astronomy-twilight-50ca0dd.jpg\" "\\"\\"\/")
There are different grades of twilight, depending on how far below the horizon the centre of the Sun\u2019s disc is. Credit: BBC Sky at Night Magazine<\/figcaption><\/figure>The magnitude of both these effects depends on latitude.<\/p>
At the equator, where the path of the Sun is perpendicular to the horizon, the Sun\u2019s apparent motion of 1\u00ba every four minutes yields a day that is roughly six minutes longer than the night.<\/p>
Moving closer to the poles, the path of the Sun relative to the horizon is increasingly slanted<\/p>
So the Sun\u2019s apparent motion in the vertical direction is slower and it takes longer to clear the horizon.<\/p>
At 50\u00baN, stargazers face an extra 10 minutes of day before the first night of autumn begins.<\/p>
Equinoxes of Saturn<\/strong><\/h2>![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/09\/Saturn-equinox-4299f29-e1599661824611.jpg\" "\\"\\"\/")
The Cassini mission’s view of Saturn during the planet’s equinox in 2009. Credit: NASA\/JPL\/Space Science Institute<\/figcaption><\/figure>Equinoxes aren\u2019t just for Earth<\/p>
All planets with a significantly tilted axis of rotation move through two equinox points as they orbit the Sun.<\/p>
But these moments are particularly spectacular on Saturn<\/a>, as its brilliant rings seem to disappear!<\/p>Saturn’s rings<\/a> are comprised of icy particles orbiting in a thin disc aligned with the planet\u2019s equatorial plane.<\/p>These are lit by the Sun throughout the Saturnian year.<\/p>
When Saturn reaches equinox, the rings are edge-on to the Sun and cast themselves into shadow, vanishing from sight.<\/p>
Saturn\u2019s equinoxes happen once every 14.7 Earth years.<\/p>
Those who observe the southern hemisphere sky<\/a> mark the advent of spring on this date, which is their vernal rather than autumnal equinox.<\/p> The term equinox is derived from the Latin for \u2018equal night\u2019<\/p> It reflects how on these dates, outside of the polar regions, the Sun will be above the horizon for as long as it is below it.<\/p> So why the extra daylight, which ranges from a few minutes at the equator to over 20 minutes nearer the poles?<\/p> Part of the answer is that we see the Sun as a resolved disc measuring 0.5\u00ba across rather than as a point source.<\/p> Astronomical convention defines the sunrise and sunset times as the first and last moments that the solar disc can be seen, rather than when its centre crosses the horizon.<\/p> Sunrise and sunset are therefore earlier and later, respectively, than the times at which the Sun is aligned with the horizon.<\/p> This \u2018extra\u2019 daytime is gained twice per day as the 0.25\u00ba between the limb and the centre of the Sun rotates through the horizon.<\/p> Daytime is extended even further by the refraction of sunlight in Earth\u2019s atmosphere.<\/p> Refraction<\/a> is the bending of waves as they travel through materials of different density, as sunlight does when entering our thick atmosphere from interplanetary space.<\/p> When we view the Sun near the horizon, we see it through a layer of atmosphere that bends its light toward us by a full 0.5\u00ba, making it appear higher in the sky than it actually is.<\/p> This means that at dawn, we see the full disc of the Sun above the horizon when it is still entirely below it!<\/p> More \u2018extra\u2019 daytime is needed for Earth to rotate through this additional angle.<\/p> The magnitude of both these effects depends on latitude.<\/p> At the equator, where the path of the Sun is perpendicular to the horizon, the Sun\u2019s apparent motion of 1\u00ba every four minutes yields a day that is roughly six minutes longer than the night.<\/p> Moving closer to the poles, the path of the Sun relative to the horizon is increasingly slanted<\/p> So the Sun\u2019s apparent motion in the vertical direction is slower and it takes longer to clear the horizon.<\/p> At 50\u00baN, stargazers face an extra 10 minutes of day before the first night of autumn begins.<\/p> Equinoxes aren\u2019t just for Earth<\/p> All planets with a significantly tilted axis of rotation move through two equinox points as they orbit the Sun.<\/p> But these moments are particularly spectacular on Saturn<\/a>, as its brilliant rings seem to disappear!<\/p> Saturn’s rings<\/a> are comprised of icy particles orbiting in a thin disc aligned with the planet\u2019s equatorial plane.<\/p> These are lit by the Sun throughout the Saturnian year.<\/p> When Saturn reaches equinox, the rings are edge-on to the Sun and cast themselves into shadow, vanishing from sight.<\/p> Saturn\u2019s equinoxes happen once every 14.7 Earth years.<\/p>What the autumn equinox 2024 means<\/strong><\/h2>
Refraction and daylight extension<\/strong><\/h2>
Equinoxes of Saturn<\/strong><\/h2>