Confused about declination, elevation & altitude

[swampwiz]

They seem to be similar terms, although elevation & altitude seem to be the exact same thing.

AIUI, the declination of astronomical object refers to the latitude on Earth where it is at the celestial zenith - i.e., straight up, along the line from the Earth's center and surface at such zenith, so this point on Earth moves along the latitude line, East to West.

What is confusing me is this Wikipedia article about Lunar Standstill. It says that during the 2006 standstill, the declination of the Moon from Sydney, Australia and London was ~29.5°. However, the Moon only has a maximum declination of 28.725°. Where did this extra 3/4 of a degree come from?

https://en.wikipedia.org/wiki/Lunar_standstill

 

[phyzguy]

The 28.725 is the maximum declination as seen from the center of the Earth. The moon is close enough to the Earth that parallax causes the moon to appear in slightly different positions when seen from different locations on the Earth. This is the source of the extra 3/4 degree. Further down in the Wikipedia article you linked, they say:

"The maximum lunar declination, as seen from the centre of the Earth, was at 01:26 on 15 September, when the declination reached +28:43:21.6. The next highest was at 07:36 on 4 April, when it reached +28:42:53.9

However, these dates and times do not represent the maxima and minima for observers on the Earth's surface.

For example, after taking refraction and parallax into account, the observed maximum on 15 September in Sydney, Australia was several hours earlier, and then occurred in daylight. The table shows the major standstills that were actually visible (i.e. not in full daylight, and with the Moon above the horizon) from both London, UK, and Sydney, Australia."

 

[swampwiz]

The 28.725 is the maximum declination as seen from the center of the Earth. The moon is close enough to the Earth that parallax causes the moon to appear in slightly different positions when seen from different locations on the Earth. This is the source of the extra 3/4 degree. Further down in the Wikipedia article you linked, they say:

"The maximum lunar declination, as seen from the centre of the Earth, was at 01:26 on 15 September, when the declination reached +28:43:21.6. The next highest was at 07:36 on 4 April, when it reached +28:42:53.9

However, these dates and times do not represent the maxima and minima for observers on the Earth's surface.

For example, after taking refraction and parallax into account, the observed maximum on 15 September in Sydney, Australia was several hours earlier, and then occurred in daylight. The table shows the major standstills that were actually visible (i.e. not in full daylight, and with the Moon above the horizon) from both London, UK, and Sydney, Australia."

So what you are saying is that closer the poles, this parallax would be even greater?

I think I can grok this by thinking about satellite dishes around 60° latitude (e.g., Scandinavia, St. Petersburg, Russia) - they are almost parallel the ground, and certainly nowhere near 30° elevation, and this is because the geosynchronous orbit around the equator (i.e., the only orbit in the sky that is viewed from an Earth station as being fixed) is at about 7X the radial distance as the Earth's surface.

 

[phyzguy]

Well, not necessarily closer to the poles. It depends where the Moon is in its orbit and where you are on the surface of the Earth. Look at this sketch. Observer O1, on a line between the Earth's center and the Moon center, sees the moon in one location, relative to the fixed stars. Observer O2, at a different location, sees the Moon in a slightly different location, relative to the fixed stars. It's the angular distance between O2 and O1 that matters. O1 would be in a location where the moon is directly overhead, which obviously changes through the month and through the day.