Different sunset time? What???

[wedding photographer]

I am going to photograph a wedding in Miami in April 2011.
I have noticed that different websites give different sunset/sunrise times! Sometimes there is one hour (!) difference... It is a lot!

For example if you look for the sunset time for Miami for 04/23/2011 on this website:
http://www.timeanddate.com/worldcloc...omy.html?n=156
or this one:
http://www.calendar-updates.com/sun....F+Sunset+Times

The sunset time is: 7:48pm

On the NAVY's website: http://www.usno.navy.mil/USNO/astron...rs-one-year-us

It is 18:48= 6:45pm!!!

Can somebody explain this to me???
I am sure others have faced this problem as well.

[cozmocha]

I think you're thinking about different kinds of sunsets.

Notice they give actual, civil, nautical... times and each is different. http://www.wunderground.com/US/FL/Miami.html

Civil Twilight

This is defined to be the time period when the sun is no more than 6 degrees below the horizon at either sunrise or sunset. The horizon should be clearly defined and the brightest stars should be visible under good atmospheric conditions (i.e. no moonlight, or other lights). One still should be able to carry on ordinary outdoor activities.

Nautical Twilight

This is defined to be the time period when the sun is between 6 and 12 degrees below the horizon at either sunrise or sunset. The horizon is not defined and the outline of objects might be visible without artificial light. Ordinary outdoor activities are not possible at this time without extra illumination.

Astronomical Twilight

This is defined to be the time period when the sun is between 12 and 18 degrees below the horizon at either sunrise or sunset. The sun does not contribute to the illumination of the sky before this time in the morning, or after this time in the evening. In the beginning of morning astronomical twilight and at the end of astronomical twilight in the evening, sky illumination is very faint, and might be undetectable.

Length Of Day

This is defined to be the time of Actual Sunset minus the time of Actual Sunrise. The change in length of daylight between today and tomorrow is also listed when available.


http://www.wunderground.com/about/faq/twilights.asp

I am going to guess the Navy uses nautical.