The planets of our solar system all revolve around the Sun in the same direction and in orbits that lie in nearly the same plane. This is strong evidence that the planets formed simultaneously from a single disk of material that rotated in the same direction as the modern planets.

Precisely when the planets came into being has been a difficult issue to resolve. While Earth’s water is necessary for life, its abundance near the planet’s surface makes rapid erosion inevitable. Continuous alteration of the crust by erosion and also by igneous (volcanic) and metamorphic (pressure and heat within Earth) processes makes unlikely any discovery of rocks nearly as old as Earth.Thus geologists have had to look beyond this planet in their efforts to date Earth’s origin. Fortunately, we do have samples of rock that appear to represent the primitive material of the solar system. These samples are meteorites, which originate as extraterrestrial objects, called meteors, that have been captured in Earth’s gravitational field and have then crashed into our planet.

Some meteorites consist of rocky material and, accordingly, are called stony meteorites. Others are metallic and have been designated iron meteorites even though they contain lesser amounts of elements other than iron. Still others consist of mixtures of rocky and metallic material and thus are called stony-iron meteorites. Meteors come in all sizes, from small particles to the small planets known as asteroids; no asteroid, however, has struck Earth during recorded human history. Many meteorites appear to be fragments of larger bodies that have undergone collisions and broken into pieces. Iron meteorites are fragments of the interiors of these bodies, comparable to Earth’s core, and stony meteorites are from outer portions of these bodies, comparable to Earth’s mantle (the layer between the core and outer crust).

Meteorites have been radiometrically dated by means of several decay systems, including rubidium-strontium, potassium-argon, and uranium-thorium. The dates thus derived tend to cluster around 4.6 billion years, which suggests that this is the approximate age of the solar system. After many meteorites had been dated, it was gratifying to find that the oldest ages obtained for rocks gathered on the surface of the Moon also were approximately 4.6 billion years. This must, indeed, be the age of the solar system. Ancient rocks can be found on the Moon because the lunar surface, unlike that of Earth, has no water to weather and erode rocks and is characterized by only weak movements of its crust.

Determining the age of the universe has been more complicated. Most stars in the universe are clustered into enormous disk-like galaxies. The distance between our galaxy, known as the Milky Way, and all others is increasing. In fact, all galaxies are moving away from one another, evidence that the universe is expanding. It is not the galaxies themselves that are expanding but the space between them. What is happening is analogous to inflating a balloon with small coins attached to its surface. The coins behave like galaxies: although they do not expand, the space between them does. Before the galaxies formed, matter that they contain was concentrated with infinite density at a single point from which it exploded in an event called the big bang. Even after it assembled into galaxies, matter continued to spread in all directions from the site of the big bang.

The evidence that the universe is expanding makes it possible to estimate its age. This evidence, called the redshift, is an increase in the wavelengths of light waves traveling through space—a shift toward the red end of the visible spectrum of wavelengths. Expansion of the space between galaxies causes this shift by stretching light waves as they pass through it. The farther these light waves have traveled through space, the greater the redshift they have undergone. For this reason, light waves that reach Earth from distant galaxies have larger redshifts than those from nearby galaxies. Calculations based on these redshifts indicate that about 13.7 billion years ago all of the galaxies would have been at one spot, the site of the big bang. This, then, is the approximate date of the big bang and the age of the universe.

我们太阳系的行星都围绕太阳向同一个方向旋转，围绕着几乎同一平面的轨道。这有力的证据表明这些行星是从单一的物质盘中同时形成的，这些物质与现代行星的旋转方向相同。 准确地来说，行星的形成是一个很难解决的问题。虽然地球的水是生命必需的，但它在地球表面丰富的水资源使得不可避免的出现快速侵蚀。通过侵蚀以及火成岩（火山）和变质作用（地球内部的压力和热量）过程不断地改变地壳，因此使得人们不可能发现几乎和地球一样古老的岩石。因此，地质学家不得不在地球起源的地方寻找到这个星球以外的地方。幸运的是，我们确实拥有似乎代表太阳系原始材料的岩石样本。这些样本是陨石，而这些陨石们来自于外星物体，称为流星，它们受到地球引力场的吸引，然后坠入我们的星球。 一些陨石由岩石物质组成，因此被称为石陨石。另一些是金属的，被指定为铁陨石，尽管它们含有少量除铁之外的元素。还有一些是由岩石和金属材料的混合物组成，因此被称为石铁陨石。流星有各种大小，从小颗粒到后来的小行星;然而，在人类历史上，没有小行星撞击地球。许多陨石似乎是经历碰撞后，碎成的碎片。铁陨石是这些天体内部的碎片，与地球的核心相当，而石质陨石则来自这些物体的外部，与地球的地幔（核心层和外壳层之间）相比较。 陨石通过几种衰减系统进行了辐射测量，包括铷-锶，钾氩和铀。由此推导其产生的日期大约在46亿年左右，这表明这是太阳系的大致年龄。在许多陨石已经过时之后，科学家们很高兴地发现，在月球表面采集到的岩石最古老的年龄大约有46亿年，事实上，这一定是太阳系的年龄。在月球上可以发现古老的岩石，因为月球表面与地球不同，它没有水和岩石的侵蚀，其特点是地壳运动性很弱。 确定宇宙的年代更为复杂。宇宙中的大多数恒星都聚集在巨大的盘状星系。我们的星系，即银河系，和其他星系之间的距离正在越来越远。事实上，所有的星系正在远离彼此，证明了宇宙在扩张，并不是星系本身在膨胀，这些硬币的表面就像星系一样：尽管它们没有膨胀，但它们之间的空间却在相互作用。在星系形成之前，它们所包含的物质在一个叫做“大爆炸”的事件中爆炸。即使在成为星系之后，物质仍然从大爆炸地点向各个方向扩散。 宇宙膨胀的证据使我们可以估计它的年龄。这种被称为红移的证据就是通过空间传播的光波波长的增加，向可见光波长的红色端移动。星系之间的空间扩大从而引起这种转变，通过拉伸光波穿过它。些光波在太空中传播得越远，它们所经历的红移就越大。出于这个原因，遥远星系到达地球的光波比附近星系的红移有更大的红移。基于这些红移的计算表明，大约137​​亿年前，所有的星系都会在一个点上，即大爆炸的地点。那么，这就是大爆炸和宇宙时代的大致日期。