As far as I know the Gayatri mantra appears in Rig Veda 3.62.10:

> 
> Om Bhur Buvaha Svaha
> 
> 
> Tat Savithur Varenyam
> 
> 
> Bhargo Devasya Dheemahi
> 
> 
> Dhiyo Yonaha Prachodayath
> 
> 
> 

But in some books and on the Internet, I've also seen a longer version. For example, the longer version is found on [this](http://vishuji.org/sandhya_vandanam.htm) page as:

> 
> Om Bhuh
> 
> 
> Om Bhuvaha
> 
> 
> Om Suvaha
> 
> 
> Om Mahaha
> 
> 
> Om Janaha
> 
> 
> Om Tapaha
> 
> 
> Om Satyam
> 
> 
> Om Tat Savithur Varenyam Bhargo Devasya Dheemahi Dhiyo Yonah
> Prachodayath
> 
> 
> Om apo jyotih raso amritam Brahma
> 
> 
> Bhur Bhuvas suvar
> Om
> 
> 
> 

What is the origin of this longer version? Does it appear in the Rig Veda?