The Sun and the planets.
Image: Paoli Smith Photography
Source: Museum Victoria
Question: I heard on the radio this week that the earth is gradually moving closer to the sun? Is this true and would it be contributing to global warming?
Answer: Over the course of the year the distance between the Earth and the Sun does vary slightly, because the Earth's orbit is not perfectly circular, it is elliptical. The Earth is furthest from the Sun in early July and is closest to the Sun in early January. So from July until January you can say that the Earth is getting closer to the Sun, and after early January the Earth is getting further away from the Sun.
However this change in distance to the Sun has no effect on the Earth's weather. The change in distance is relatively small, only a little over 1% of the average distance to the Sun. Seasons on Earth are caused by the changing patterns of sunlight caused by the tilt in the Earth’s axis, not by this small change in distance to the Sun. Furthermore, the average distance to the Sun does not change from one year to the next, so this small change in distance has no effect on climate.
The shape of the Earth's orbit does change over many thousands of years and can become less circular than it is today. When this happens, the variation in distance to the Sun can have an impact on the Earth's climate. In fact, this change is responsible for regulating the cycle of ice ages on Earth. But this cycle occurs over tens of thousands of years, and is not the cause of recent climate change.
Also, over a very long time frame, the Sun is getting hotter and hotter. This is another possibility for what the person on the radio was talking about. Indeed, over many millions of years, this is the strongest influence on climate on Earth. In around a billion years, the heat of the Sun will probably make the Earth uninhabitable. However this is a very slow process and is certainly not the cause of the relatively rapid change in climate over the last century.
There are a number of other influences of the Sun that some people have tried to use to explain changes in the climate, such as the sunspot cycle, or the movement of the solar system through the galaxy. To date none of these explanations has been able to explain recent climate change to the satisfaction of the broad scientific community.