In this quote from Chapter Seven, Tom makes an inaccurate statement about the sun by saying that it is getting hotter every year and that the Earth might fall into it. He then contradicts himself by saying the opposite: that the sun is actually getting colder.
This is significant because it demonstrates Tom's arrogance. He is not really interested in the facts about the sun; it doesn't really matter to him whether the sun is getting hotter or colder. He is more interested in making a public statement and in being perceived as cultured and educated.
In addition, this inaccuracy also demonstrates his competitive nature. Remember that Gatsby is present during this scene and he senses the attachment between Gatsby and his wife. As such, Tom wants to be noticed, more so than Gatsby, and by quoting facts about the sun (even incorrect ones), he makes sure that he is at the forefront of the conversation.
I think that the following quote from Tom refers to his view on his life. He knows that his wife is in love with Gatsby. Tom who once felt like the center of the universe is losing both his wife and his mistress.
Fitzgerald also uses the heat of the summer to symbolize the passion of these characters. It is the hottest day of the year when Gatsby and Tom have their grand confrontation about Daisy.
“I read somewhere that the sun’s getting hotter every year,” said Tom genially. “It seems that pretty soon the earth’s going to fall into the sun—or wait a minute—it’s just the opposite—the sun’s getting colder every year." (Fitzgerald)
It could also be a reference to the loss of heat or passion for his lifestyle. Every year, the booze infused partying gets more and more boring.
In Chapter 7, the heat has heightened Tom's confusion. He feels like his life is spinning out of control. He is no longer the center of attention in Daisy's world, he has witnessed his wife display her love for Gatsby.
"There is no confusion like the confusion of a simple mind, and as we drove away Tom was feeling the hot whips of panic. His wife and his mistress, until an hour ago secure and inviolate, were slipping precipitately from his control."