Equinox

[eosophobiac]

While looking at my weather page/almanac page, I noticed that tomorrow in my location, the sun will rise and set at the same time on the clock, just twelve hours apart. That is, 6:14 am and 6:14 pm (southern Maryland). Then I punched in a couple of other locations and saw that they had equal time for different dates. One was 6:51 am and pm for <b>today</b> (San Angelo, TX), and one was 6:09 am and pm for tomorrow as well (Mohawk, NY). <br />What I'm wondering is if Spring is officially Monday, why are the equinoxes not occuring closer to Monday? Or why isn't the first day of spring occuring earlier? Is the first day of Spring an average of all the 'local' equinoxes?<br />Also, is the first day of Fall - for the Southern Hemisphere - on Monday as well? <img src="/images/icons/smile.gif" /><br /> <br />Although not strictly an astronomy question, I thought I'd get better a response here. Feel free to move this thread if needed. <br /><br /> <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[tfwthom]

I'm pulling this from my (faulty) memory so.....<br /><br />Seems that I remember reading somewhere that the "North Star" was/will be Vega and/or Capella. The shift in the "North Star" happens over centuries. Calendars (date) of the equinox do not take the shift into effect. So since the position of the pole determents where the Sun rises or sets the date/time of the equinox changes. They don't want to confuse everyone by changing the date. It's like why we add a couple seconds so the sun is directly over head at noon. (like we added this year)<br /><br />Best link I could find about it. http://solar.physics.montana.edu/YPOP/Classroom/Lessons/Sundials/equinox.html<br /> <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[CalliArcale]

The "official" equinox is the astronomical equinox. Oddly, it does not take a whole day, but is a single instant in time. How, then, can day and night be equal on the equinox, if it doesn't last a whole day? Basically, the day and night closest to the astronomical equinox will be as close to equal as makes no difference. Depending on your latitude, the days surrounding the equinox may be very close to equal as well; changes in the length of the day are progressively more extreme the farther north or south you go. (Consider also that sunrise and sunset are also determined by elevation, terrain, and time zone.)<br /><br />Consider the imaginary line of the Earth's rotational axis. Imagine you are looking at it from the perspective of the Sun. At a solstice, you will see it as if it is perfectly vertical. It isn't, of course, but from this point of view it seems to be. As the year progresses, the axis will seem to tilt more and more until eventually it is tipped 23.45 degrees. Then it will right itself, going back to vertical. It will continue on, tipping now in the other direction until it is tipped 23.45 degrees the other way. It will reverse and go back towards vertical again. This is what happens over the course of one year.<br /><br />When the Earth's axis appears to be tilted the most is the equinox. It is when a viewer on the Sun would see exactly the same amount of Earth both above and below the Earth's equator, and when the imaginary line of the Earth's axis appears most tilted. As a consequence of this arrangement, the Sun will rise due east and set due west from absolutely any location on Earth. It is the only day of the year when this is true; at the equator, the Sun's path doesn't change very much over the course of the year, but as you go further north or south, the difference becomes very pronounced. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

BTW, the (northern) autumnal equinox is on a Saturday this year, as least on my calendar: Sept 23. It is not always the same date, which may seem odd, but this is because the astronomical equinox is a single instant in time, and the number of days in a year is not a whole number. It may even fall on different dates for people in different time zones. For instance, if it occurs at 1:02AM on March 20 in Washington, DC, it will occur at 10:02PM, March 19 in Los Angeles.<br /><br />I'm not sure about this, but I think the date is defined by what date it will be in UTC (i.e. in Greenwich, England, which defines the Prime Meridian) at the instant of equinox. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[eosophobiac]

I wondered if the Prime Meridian figured into this at all. I seemed to remember reading somewhere that the equinox isn't the exact same time for all locations also, but couldn't find where I'd read it. <br />I'd been googling/following links since I posted the question, and some of what I found was in terminology I couldn't quite get me head around! Thanks for using lingo I can understand.<br /><br />And TFWThom - thanks for the link. That helped me visualize even better than a few others I had come across.<br /><br /><br /> <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[tropicalzone]

actually it because of refraction daylight for most parts of earth on equinox is 12hr 8min so for a place at 50N 12hr is on mar 17/18 and sept 25/26