Spritesheet counting and controlling problems

Friends, I´m having a small problem here

I´m making an app to show moon phases. 

There are 8 images of moon inside a spritesheet, but I´m struggling to change the images according to the moon.

I need to link a moon cycle variable to the sprite sheet, so it will change the image of the moon at aproximately every 3 days. But since spritesheets seems to be a closed method, how can i pass my conditions externaly to increment sprite count?

Are there any way to do that or shall i use a sequence of images instead of a spritesheet?

Thank you for any help!

Bellow a piece of my code:

...mooncycle calculation code... M = Mooncycle variable --(between 0 and 3 would be new moon, between 3.1 and six crescent moon...and goes on until it repeats again) --Starting graphical block background = display.newImageRect("MoonBack.png",320,480) background.x = display.contentCenterX background.y = display.contentCenterY cabecalho = display.newText( "Moon Phases", 160, 5 , native.systemFontBold , 20 ) cabecalho:setFillColor(0, 0.5, 0.8) --building spritesheet local sheetData = { width=160, height=203, numFrames=8, sheetContentWidth=1280, sheetContentHeight=203 } local FaseMoon = graphics.newImageSheet("Lunarp.png", sheetData) local sequenceData = { {name="FasesLua", start=1, count=8, loopCount=0, loopDirection="forward" } } local animation = display.newSprite(FaseMoon,sequenceData) animation.x=display.contentWidth/2 animation.y=display.contentWidth/2.3

I can see the first frame of my spritesheet, but how can i increment the frame counting according to the lunar cycle?

Thanks !