TweenService works great to smoothly interpolate properties of instances from a start to a finish, but what if you wanted to do something like make a part smoothly orbit around another? You couldn’t directly use TweenService because it would just connect the start position to the goal position by a straight line and move along it.
The trick to get around this is to use ‘Value’ instances to store a datatype which you are able to Tween, and then connect a function to it’s .Changed event so that you can use the Value as a parameter as it’s being tweened.
Here’s an example where I tweened a NumberValue to be used as the angle for a parameterized orbit to make a star spiral into a black hole 
:
(directories)
(script)
Source Code
local TweenService = game:GetService("TweenService")
local blackhole = script.Parent
local angle = blackhole.Angle
local star = workspace.Star
local angleInfo = TweenInfo.new(6,Enum.EasingStyle.Quad,Enum.EasingDirection.In,0,false,0)
local angleTween = TweenService:Create(angle, angleInfo, {Value = 10*math.pi})
angle.Changed:Connect(function(value)
local radius = 40*(10*math.pi - value)/(10*math.pi)
star.Position = blackhole.Position + radius*Vector3.new(math.cos(value), 0, math.sin(value))
end)
blackhole.ClickDetector.MouseClick:Connect(function()
star.Position = blackhole.Position + Vector3.new(40,0,0)
angleTween:Play()
end)
Just set up your tween for the Value instance, and then connect your function to the it’s .Changed event, and that’s it. I used a click detector to activate the tween but that’s not important here
hope this helps 