Читать книгу Exploring the Solar System - Peter Bond - Страница 67

Earth is the third planet from the Sun. It follows an almost circular orbit (eccentricity 0.017) at an average solar distance of 149,600,000 km. At perihelion, which occurs around January 3, the Sun–Earth distance is 147.1 million km. Aphelion, when Earth is farthest from the Sun, is reached around July 4. The planet is then 152.1 million km from the Sun. Moving at a mean orbital speed of about 30 km/s, Earth completes each orbit in one year or 365.25 days.

The planet rotates from west to east once every 24 hours, on average.¹ This period, from noon to noon, is called the solar day. As a result, Earth's equatorial region is moving eastward at a rate of 1,670 km/h, though this rate decreases to almost nothing near the poles.

The planet's rotation explains why the Sun, Moon, and stars appear to rise in the east and set in the west. However, the time of sunrise and sunset changes by up to 4 minutes each day because the planet is also moving around its orbit. This means that Earth has to rotate for approximately another 4 minutes before the Sun returns to the same place in the sky where it was the previous day. On the other hand, Earth's sidereal day, measured relative to the position of the “fixed” background stars, is 23 hours 56 minutes 4 seconds (see Figure 3.1).

Earth's rotational axis is currently tilted 23.44° to the ecliptic plane. The axis is aligned in the same direction relative to the stars throughout the year, with the North Pole pointing approximately in the direction of the star Polaris.

However, the orbit and rotation rate are not stable and unchanging. The planet's motion is influenced by its gravitational interaction with the Sun, Moon, and other planets. There are four major variations over time:

 The shape of Earth's orbit slowly changes from almost circular to more elliptical on a regular basis. Astronomers say that its eccentricity changes from nearly zero to 0.06, and then back again, in a cycle that takes between 90,000 and 100,000 years. When the orbit is more elliptical, the amount of insolation (incoming solar radiation) received at perihelion is about 0.2% greater than at aphelion.

 The direction in which Earth's axis points changes slowly because the planet wobbles on its axis as it spins – an effect known as precession. The cause of the precession is the planet's equatorial bulge, caused by Earth's fairly rapid rotation. The pull of the Moon and Sun on the bulge makes the Earth precess. This wobbling motion affects the direction in which the rotational axis is inclined, but it does not affect the obliquity (tilt angle) of Earth. The planet completes a full wobble every 25,800 years. At present, the rotation axis is pointing almost exactly at Polaris. However, 13,000 years from now the axis will be aligned toward Vega, the brightest star in the constellation Lyra (see Figure 3.2a).Figure 3.1 On a prograde planet like Earth, which rotates from west to east, the sidereal day is shorter than the solar day. At 1, the Sun and a certain distant star are both overhead. At 2, the planet has rotated 360° and the distant star is overhead again. The time period between 1 and 2 is one sidereal day. Only when Earth has rotated a little further is the Sun overhead again (3). The interval between 1 and 3 is one solar day.(Wikipedia)

 Another type of precession is associated with the sideways rotation of Earth's elliptical orbit. Known as precession of the ellipse or orbital precession, this motion causes the positions of perihelion and aphelion to shift with regard to the stars.

 The obliquity or inclination of Earth's axis, i.e. its angle of tilt relative to the plane of its orbit, varies between 22.1° and 24.5° every 41,000 years. (It is currently decreasing). Such changes modify Earth's seasons. A higher inclination means more extreme seasons – warmer summers and colder winters. A lower inclination results in less severe seasons – cooler summers and milder winters (see Figure 3.2b).

All of these changes affect the timing and duration of the seasons. Today, northern summer is the longest season and northern winter the shortest; 10,000 years from now, the length of the seasons will be reversed. The gradual modifications of the orbit are also thought to explain Earth's periodic ice ages (see Ice Ages).

Figure 3.2 (a) Earth rotates (white arrows) once a day around its rotational axis (red). Like a spinning top, Earth's axis wobbles as it rotates. This effect, known as precession, causes the axis to describe a circle in space (white circle) once every 25,800 years. As time goes by, the planet's North Pole points toward different stars. (Robert Simmon, NASA‐GSFC) (b) The change in the tilt of the Earth's axis (obliquity) affects the magnitude of seasonal change. At higher inclinations the seasons are more extreme, and at lower inclinations they are milder. The current axial tilt is 23.44°.

(Robert Simmon, NASA‐GSFC)