A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.However, by itself a fossil has little meaning unless it is placed within some context.Comparing the mass and luminosity of the Sun to those of other stars, it appears that the Solar System cannot be much older than those rocks.Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old, giving an age for the Solar System and an upper limit for the age of Earth.By contrast, absolute dating allows us to assign dates to geological features.To avoid confusion later on, let us say at once that the "absolute" in "absolute dating" is not short for "absolutely correct".
Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine.
Suppose that we wanted to find out how long it has been since an hourglass was set running by measuring the amount of sand in the lower bulb.
To do so successfully, we would need to assure ourselves of the following conditions: Given these conditions, we can find out how long the hourglass has been running.
If we wish to use a geological process as the geological equivalent of an hourglass, we would want to have similar conditions: we would like to find some quantity which we can measure reliably (corresponding to the condition that we can measure the amount of sand in the lower bulb of the hourglass); which increases or decreases from a known quantity (corresponding to the lower bulb of the hourglass being empty when it starts running) at a known rate (corresponding to knowing the rate of flow of sand); and so forth.
Note that the conditions we have given for the hourglass are ideal conditions which we would require to know exactly how long it is since the hourglass started running.