lunar data study

Posted 2023-04-01

参考技术A

大地测量基准是由一组确定测量参考面（参考系）在地球内部的位置和方向，以及描述参考面形状和大小的参数来表示。
一般选择一个椭球面作为计算的参考面。同时地球作为宇宙空间的一个行星，也有重要的物理性质

  美国国防部建立，先后有WGS-60，WGS-66，WGS72，WGS84这几个版本，世界大地坐标系WGS属于协议地球坐标系CTS，WGS可看成CTS的近似系统。 WGS84是目前GPS系统广播星历和NIMA精密星历的参考框架。
  在全球定位系统中，为了确定用户接收机的位置，GPS卫星的瞬时位置通常应化算到统一的地球坐标系统。在GPS试验阶段，卫星瞬间位置的计算采用了1972年世界大地坐标系(World Geodetic System ——WGS-72)，1987年1月10日开始采用改进的大地坐标系统WGS-84。
  WGS-84大地坐标系的几何意义：原点位于地球质心，Z轴指向BIH1984.0定义的协议地球极(CTP)方向，X轴指向BIH1984.0的零子午面和CTP赤道的交点，Y轴与Z、X轴构成右手坐标系。对应WGS-84大地坐标系有WGS-84椭球。

说明:“G”表示这些坐标是完全采用GPS方法所确定出来的，没有包 含多普勒数据;而跟在后面的数字所表示的是，在NIMA精密星历估 计过程中，开始使用这些坐标时的GPS周数，如，数字“873”是历元 1996年9月29日0hUTC时的GPS周数，而数字“1150”是历元2002年 1月20日0h，从这些天起，用户可以通过NIMA的GPS星历来分别使 用WGS 84 (873)和WGS 84 (1150)。

  在日月引力等因素的影响下，瞬时北天极将绕瞬时平北天极产生旋转，轨迹大致为椭圆。这种现象称为 章动 (Nutation)。

为了将协议天球坐标系的卫星坐标，转换为观测历元t的瞬时天球坐标系，通常分两步进行。

由于地球上一固定点在天球坐标系中的坐标随地球自转而变化，应用不方便。为了描述地面观测点的位置，有必要建立与地球体相固联的坐标系—地球坐标系。地球坐标系与地球固连在一起，随地球一起自转，又称 地固坐标系 。 地球坐标系的主要任务是用以描述地面点在地球上的位置，也可用以描述卫星在近地空间中的位置。

Earth-Centred Earth-fixed Coordinate System 地球体相固联的坐标系统，英文简写（ECEF，CTS） ，又称地固坐标系。

协议地球坐标系CTS又称为国际地球参考系统(ITRS)。

在GPS测量中，经常要进行坐标变换和基准变换。

开普勒方程

听力Lunar Eclipse

月食知多少 来自英文巴士 00:00 03:01

NB: This may not be a word-for-word transcript.

Lunar Eclipse 101

A lunar eclipse happens when the Earth blocks some or all of the Sun’s direct light from reaching the Moon. This cosmic event only takes place during a full moon, which happens once every 29 and a half days, or the length of one full orbit of the Moon around the Earth. 

So, why don’t we have any eclipse every month?

The Moon’s orbit is tilted a few degrees in relation to the Earth, so the Earth, Moon and Sun don’t always align. When the Earth does eclipse the Sun, it casts two types of shadows on the Moon – a larger shadow known as the penumbra, and a smaller darker shadow known as the umbra. 

There are three types of lunar eclipses. The first is a total lunar eclipse, when the Sun, Moon and Earth are in perfect alignment and the Moon falls within the Earth’s umbral shadow. Total lunar eclipses are the most striking of the three types because they turn the Moon a sunset red while shorter blue wavelengths of light are scattered outward by the Earth’s atmosphere, longer red wavelengths are refracted or bent inward toward the Moon, making it appear red. The brightness of the Moon’s red glow depends upon how much dust and clouds are in the Earth’s atmosphere. Following volcanic activity, ash can block out enough light to render the Moon a darker red or even mere black. 

A partial lunar eclipse – the second type – occurs when the Earth, Moon and Sun don’t perfectly align so only part of the Moon passes into Earth’s umbra. Earth’s shadow appears very dark on the side of the Moon facing Earth.

Last, a penumbral lunar eclipse occurs when the Moon passes through Earth’s penumbral shadow. The event is so subtle that most people don’t even notice. The moon will appear just slightly darker than normal.

Lunar eclipses occur up to three times a year and can be observed from the entire night time half of the Earth. Unlike during a solar eclipse, it’s safe to look at the Moon with the naked eye during a lunar eclipse. It is only because of the distances of the Sun and Moon from the Earth that we are able to witness total lunar eclipses.

As the Moon inches away from the Earth each year, one day, billions of years from now, the Moon will be too far away to fall completely within the Earth’s umbral shadow. Until then, we can occasionally enjoy seeing our own planet’s shadow cast upon the largest object in the night’s sky.