In 2015 we learned a lot about the Solar System and beyond. And the splendid sky events included a solar eclipse and two lunar eclipses. Yet, as ever, people on social media who delight in disaster were declaring doom. Should we be apprehensive? Let's see.

Some notes on gravity
The four predictions of astronomical non-events that I've chosen have problems with gravity, so we'll start with Isaac Newton.

Newton showed that the strength of a gravitational force is related to mass and distance. The more massive a body is, the more force it exerts. But if its distance from us increases, this force rapidly decreases. Imagine two bodies with the same mass, but one is twice as far away from us as the other. The force of the more distant one wouldn't be half that of the nearer one, it would be a quarter. If it were ten times farther away, its force on us would be a hundred times less.

Now think of tides on Earth. Both the Moon and the Sun affect them. However most of the effect is due to the Moon's gravity. The Sun is massive, but it's about 400 times farther away than the Moon, so the Moon's gravitational pull on Earth is much stronger. Yet it's not strong enough to set off earth tremors and volcanic eruptions.

1. Comet or asteroid impact
A web story's heading read: END OF DAYS? 2.5 Mile Wide Comet Expected To Impact Earth September 15-28, 2015. It continued, “According to a credible retired NASA astronomer, a 2.5 mile wide comet is expected to impact Earth in September 2015.” (The astronomer wasn't named.)

This prediction evolved from a warning given in 2010 by a minister who'd had a message from God saying that an asteroid was headed our way, and it would “soon be seen in the alarm systems of NASA”. It would land in the ocean near Puerto Rico, the ensuing earthquake and tsunami destroying the east coasts of North and South America. He didn't give a date, but by now “soon” must be over.

However on September 24 an asteroid – around 200 m (650 ft) across – passed by. As predicted by astronomers, the closest it came to Earth was over twenty times the distance to the Moon.

2. The Christmas Eve asteroid
The Sunday Express, an English paper, headlined: First view of mega asteroid so BIG 'it could trigger EARTHQUAKES' on way to Earth for Xmas. It seems that some people insisted that on Christmas Eve, asteroid 2003 SD220 would come close enough to trigger strong earthquakes and volcanoes. In fact, as the asteroid is considerably smaller than the Moon and was 25 times farther away, nothing happened. (Some way into the Express article, it did finally mention that there was no evidence that the asteroid pass would have any effect on Earth.)

3. Beware the supermoon
Since the Moon's orbit isn't quite circular, its distance from us is somewhat variable. At its most distant, the Moon is at apogee, and when it's nearest it's at perigee. Tidal effects on Earth are boosted at full and new Moon by the gravity of the Sun and the Moon working together, but the effect of distance is slight.

An astrologer invented the term supermoon, meaning a full (or new) Moon at perigee. In 2011 the astrologer proclaimed to the media that in the week centered on the “supermoon”, there would be an increase in severe earthquakes and other disastrous events.

In 2015, the September equinox full Moon at perigee was just 300 km farther away than the 2011 “supermoon”. However as in 2011, there were no seismic calamities.

4. Nibiru
I won't even try to unravel the many strands of Nibaru/Planet X, which is supposedly a large planet. Or a brown dwarf (a failed star). Or maybe a really big comet. It's a popular disaster which has been adopted by assorted doom merchants.

As Planet X, it emerged in the mid nineties. The warning that this planet was on a collision course with Earth didn't come from an anonymous “credible retired astronomer”. An Earth woman is in telepathic contact with aliens from the star system Zeta Reticuli. Planet X, they say, will pass so close to Earth that Earth's axis will “flip over”. (That would, in fact, need a massive impact, not a flyby.)

And when will this happen? In May 2003. Oh? Well, apparently either the Zetas are practical jokers or it was, as their earthly contact said, just a "white lie ... to fool the establishment." (Most – but not all – of her followers abandoned the cause at that point.)

After 2003, various people adopted the idea, but attached it to the fictitious Nibiru. Many expected it to fulfill the supposed Mayan prophecy in December 2012. Or maybe May 2013. No doubts about 2015 for the website that proclaims “this is sure to be the year for some serious signs of the approaching celestial body.” Another agrees that “Nibiru will appear in December 2015 and pass over at the end of April 2016.”

As of the end of December, still no sign of Nibiru.

So are we safe?
When misinformed people sow alarm and confusion, it's irresponsible. Nonetheless it's true that we are not safe. There is increasing evidence of the existence of large asteroids and comets in greater numbers than had been previously thought. We need to know where they are and be able to deal with them, if necessary. Many prominent individuals signed the Asteroid Day Declaration in June 2015 to ask that governments, organizations and individuals commit resources and support to accomplish these aims.

For more information about Asteroid Day, click under this article “Join the discussion” to link to a Forum page.