Ecliptic Plane

If the sun's path is observed from the Earth's reference frame, it appears to move around the Earth in a path which is tilted with respect to the spin axis at 23.5°. This path is called the ecliptic. It tells us that the Earth's spin axis is tilted with respect to the plane of the Earth's solar orbit by 23.5°. Observations show that the other planets, with the exception of Pluto, also orbit the sun in essentially the same plane. The ecliptic plane then contains most of the objects which are orbiting the sun. This suggests that the formation process of the solar system resulted in a disk of material out of which formed the sun and the planets. The 23.5° tilt of the Earth's spin axis gives the seasonal variations in the amount of sunlight received at the surface. 如果从地球的参考系观察太阳的路径，太阳似乎在地球周围沿一个倾斜于地球自转轴的路径运动，倾斜角度为23.5°。这个路径称为黄道。这表明地球自转轴相对于地球绕太阳公转平面倾斜了23.5°。观测显示，除了冥王星外，其他行星的轨道基本上都在同一平面内。因此，黄道平面包含了大部分绕太阳运行的天体。这表明太阳系的形成过程导致了从盘状物质中形成了太阳和行星。地球自转轴的23.5°倾斜导致了地表接收到的阳光量的季节性变化。

Pluto 's orbit is exceptional in that its orbit makes an angle of 17° with the Earth's orbit. This has led to a number of theories about Pluto's origin. Mercury is the only other planet which moves significantly away from the ecliptic plane ( 7°) .

The Ecliptic 黄道 The apparent path of the Sun's motion on the celestial sphere as seen from Earth is called the ecliptic. The ecliptic plane is tilted 23.5° with respect to the plane of the celestial equator since the Earth's spin axis is tilted 23.5° with respect to its orbit around the sun. The ecliptic plane intersects the celestial equatorial plane along the line between the equinoxes. 从地球上看，太阳在天球上的 apparent path 被称为黄道。黄道平面相对于天赤道平面倾斜 23.5°，因为地球自转轴相对于其绕太阳公转的平面倾斜 23.5°。黄道平面与天赤道平面相交于二分点之间的线。

The tilt of the Earth's axis with respect to the ecliptic is responsible for Earth's seasons.

Declination and Right Ascension 赤道坐标系 Right ascension and declination are like longitude and latitude on the surface of the Earth except that they are measured with respect to the celestial sphere with the vernal equinox as the origin. The right ascension and declination for a celestial object constitute a unique designation of its position, and from them one can determine the observer coordinates necessary to find the object in their frame of reference. 天体的赤经和赤纬构成其位置的唯一标识，从这些数据可以确定观测者坐标，从而在他们的参考系中找到该天体。 右升和赤纬类似于地球表面的经度和纬度，只是它们是相对于天球测量的，以春分点作为原点。

Right ascension is a celestial longitude measured in the direction of the Earth's rotation. Since that rotation makes a complete circle in 24 hours, the notation adopted for right ascension was in terms of hours and minutes with 24 hours representing the full circle. Declination is expressed as an angle with respect to the celestial equator. For example, the celestial coordinates of the star Betelgeuse in the constellation Orion are right ascension = 5 hours 52 minutes and the declination is 7 degrees 24 minutes. A declination south of the celestial equator is given a negative sign.

Equinoxes and Solstices 春分和夏至 The points where the ecliptic crosses the equatorial plane of the celestial sphere are called equinoxes. On those dates there are 12 hours each of daylight and dark. The most northern excursion of the sun is called the summer solstice and will have the longest amount of daylight. The winter solstice opposite it is the shortest period of daylight. 黄道平面与天球赤道平面的交点称为分点。在这些日期，白天和黑夜各为12小时。太阳最北端的点称为夏至，此时白昼最长。相反的冬至则是白昼最短的时候。

"At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and longitude = 0¡) and the autumnal point (RA = 12h 00m 00s and longitude = 180¡). The equinoxes are the only times when the solar terminator is perpendicular to the Equator. As a result, the northern and southern Hemispheres are equally illuminated." From Wiki.

Celestial Measurement

Celestial measurement is taken to mean the kinds of measurement with respect to the celestial sphere which allow you to locate astronomical objects for observation. Measurements with respect to the celestial sphere are typically expressed as right ascension and declination, measurements analogous to latitude and longitude on the Earth's surface. 天体测量是指相对于天球的测量类型，这些测量允许你定位天体以供观测。相对于天球的测量通常表示为赤经和赤纬，这些测量类似于地球表面上的纬度和经度。

In order to know where to look in the sky for a given object, a precise description of the Earth's motion around the sun and precise measurement of time are necessary. It is often more convenient to describe the apparent motion of sun and stars with respect to the Earth as if it were fixed. In the Earth's frame of reference, the Sun's apparent path is called the ecliptic. The ecliptic plane is inclined at 23.5° with respect to the celestial equator because of the tilt of the Earth's rotation axis with respect to the plane of its orbit around the sun.

Synodic and Sidereal Periods

The period of a planet's orbital period around the Sun with respect to the distant stars is called its sidereal period. The sidereal period of the Earth is about 365 1/4 days. Another type of period is useful for viewing the other planets - the period between the times their positions both lie on the same radial line from the sun, called the synodic period. When planets are on the same radial line from the sun, they are said to be "in opposition". For planets closer to the sun than the Earth, the synodic period of the Earth is longer than the sidereal period, and for outer planets it is shorter if seen by an observer on those planets.

The sidereal period of Mars is 1.88 years, whereas the synodic period is 2.135 years as seen from the Earth. The time of opposition of Mars is associated with its apparent retrograde motion for an Earth observer.

The period which brings the Earth back to the same angular position with respect to the Sun is called the tropical year and is 365.242 mean solar days. Formally this period is defined as the interval of time from one vernal equinox to the next. The sidereal period (period with respect to the distant stars) of 365.256 mean solar days is about 20 minutes longer because of the precession of the Earth's spin axis. That precession period of about 26,000 years brings the vernal equinox about 20 minutes earlier each year. The fact that the year is not exactly 365 days has led to the inclusion of the leap year days and other adjustments to the calendar.

The sidereal day, which brings a "fixed" star back to the same position on the next night, is 23 hours 56 minutes and 4 seconds. The practical observable effect is that stars rise about 4 minutes earlier each night, about 2 hours earlier in a month, and appear as a parade that progresses westward across the night sky.