Photos 3. Members Tosha M. Edward K. Xian M.
Steven H. Blue E. The mean ecliptic is defined by the barycenter of Earth and the Moon combined, so the Earth wanders slightly above and below the ecliptic due to the orbital tilt of the Moon. Because of the precession of the Earth's axis , the position of the vernal point on the celestial sphere changes over time, and the equatorial and the ecliptic coordinate systems change accordingly.
Thus when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date. The upper culmination of the vernal point is considered the start of the sidereal day for the observer. The hour angle of the vernal point is, by definition, the observer's sidereal time.
The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional harvest festivals are celebrated on the date of the equinoxes. One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the Sun goes directly behind the satellite relative to Earth i. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit.
The duration of those effects varies but can range from a few minutes to an hour. For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration "Sun outage" windows. Satellites in geostationary orbit also experience difficulties maintaining power during the equinox, due to the fact that they now have to travel through Earth's shadow and rely only on battery power.
Usually, a satellite will travel either above or below the Earth's shadow due to its shifted axis throughout the year; during the equinox, since geostationary satellites are situated above the equator, they will be put into the shadow of the Earth for the longest period of time all year.
Equinoxes occur on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places its ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above — a view seen during an equinox for the first time from the Cassini space probe in — they receive very little sunshine , indeed more planetshine than light from the Sun. Saturn's most recent equinox was on 11 August , and its next will take place on 6 May Mars's most recent equinox was on 23 March northern spring , and the next will be on 8 April northern autumn.
From Wikipedia, the free encyclopedia. Astronomical event where the Sun is directly above the Earth's equator. This article is about an astronomical event. For the celestial coordinates, see Equinox celestial coordinates. For the article about a specific equinox, see March equinox and September equinox.
For other uses, see Equinox disambiguation. Not to be confused with solstice. Diagram of the Earth's seasons as seen from the south. Far left: June solstice. Main article: Sun path. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. December Learn how and when to remove this template message.
See also: Equinox celestial coordinates.
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Technical references since the beginning of the 20th century c. Pitman, The earliest confirmed use of the modern meaning was in a post on the Usenet group net. Use of this particular pseudo-latin protologism can only be traced to an extremely small less than six number of predominently US American people in such online media for the next 20 years until its broader adoption as a neologism c.
Retrieved 18 September Retrieved 21 December Astronomical Information Center. United States Naval Observatory. Archived from the original on 25 May Retrieved 9 July On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth.
However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight.
At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Moon Wisdom Transform your life using the Moon's signs and Lisette Cheresson Andrea Rice.
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