Right, to all those who believe Planet Nibiru (or Planet X) will hit us in around 24 hours, lets look at a little fact shall we? Niburu is claimed to be 4x the size of Earth.
Now...that's a fucking BIG object...On that basis, you WOULD be able to see it, very clearly by now, with the naked eye...
In fact, it would be very hard to miss...
One way to look at it...is the moon is 1/4 the size of the Earth's diameter, and 238,000miles from the Earth.
That's a LONG way away. Even at that size, and that distance, it is VERY visible with nothing but the naked eye...on clear nights you can even clearly see definition such as craters.
So...A little bit more Math here...If the moon is 1/4 the Earth, that makes the Earth 4x the size of the moon. Now 4x that AGAIN...That's 952,000miles...That's how far it could be away right now, and be as visible as the moon...
Now...Travelling at the speed of light...It would take appx 1.5seconds to get from the moon to the Earth...
So, we have some basis for Math right now...
We know 'Niburu' is not visible AT ALL, so we can safely assume it is around 19,040,00Miles away (~8x the distance of the moon) - At 4x the size of the Earth, it would be perfectly visible by Telescope...but lets imagine nobody has those shall we? OK...so...the fastest meteor goes around ~160,000 mph (Very fucking fast)...That would mean...the distance it would have to be at...RIGHT NOW...on a collision course...(Not possible at the distance it would need to be)...is...3,840,000...Yes...almost 4 MILLION MILES...
BUT...what's more...is that, even in 1927, WE COULD SEE THINGS THAT FAR AWAY...A newspaper story from Wednesday 25 May 1927: http://trove.nla.gov.au/ndp/del/article/36046580
Clicked the link and read it? Good...lets carry on shall we?
Oh look...it means we would know...
So...All in all...in order for nobody to have seen it...AT ALL...no glimpse, no slight piece of solid evidence (Even amateur astronomers would be able to track at this distance)....it would have to currently be sitting, around....the 50 or 60 MILLION mile mark...OH...no it wouldn't...because guess what...MARS is 60 Million Miles away at one of its closest points to Earth...and ... hmm...guess what...ITS VISIBLE from that distance, even with the naked eye! -
SO...Lets go deeper shall we?
Pluto is 3,674,490,959miles away...o.0...that's 3.5 BILLION MILES...Btw...we can SEE that with some of the better telescopes we have...(How the fuck else would we know it's there?)...
Soooooooooo..........
Going on another basis, that NASA currently track objects within 120 million miles of us...so if it was close...we would know - BUT, if it is going (EXTREMELY) fast...and is around, for the sake of argument...5 Billion Miles from us (Other side of Pluto)...it would have to be travelling at around 138888888.8888889MPH (138.8 MILLION MPH) (38,580 Miles Per Second...or around 1/5th The Speed of light, which is approximately 186,282 miles per second)
Anyway...My point is...NO object could be travelling, that fast, and be that far away (Don't forget , if its going that fast...and the Mayans predicted it...lets say, around the end of when they where here...about 900AD...or...1112 years ago...'Planet Niburu' would have been...assuming it has always been travelling in the same direction...with no deviation (It's going fucking fast...so wtf could alter its course?) ... 1,352,933,000,000,000 Miles from Earth (or)...
1 Quardrillion, 352 Trillian, 933 Billion Miles...
Oh, I'm also ignoring the fact that the Mayans did NOT know about the Leap Year...
Anybody want me to take that into account and do some more math?
(Thanks to Deozaan for help with some of the Math in here!)
I think it's like 225,291 to 251,910 miles from the Earth depending on its elliptical orbit at the time. 
- carpe noctem
