I've come across two versions of the Gayatri mantra.
One is this:
The other is this:
The only difference is that in the first pada, the word "varenyam" is different. Both these variations are valid since they come from authoritative sources.
But why is there a difference? And which one is to be used in sandhyavandana japa?
This is what is said on wikipedia:
Whereas in principle the gāyatrī mantra specifies three pādas of eight syllables each, the text of the verse as preserved in the Samhita is one short, seven instead of eight. Metrical restoration would emend the attested tri-syllabic vareṇyaṃ with a tetra-syllabic vareṇiyaṃ.
But then why do authoritative sampradayas enjoin the first version?